Enable warnings as errors when running tests

[parthea]

There are warnings in the build logs. We should treat warnings as errors so we know when new warnings appear and create tracking bugs to resolve the existing warnings.

=============================== warnings summary ===============================
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/xdist/plugin.py:252
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/xdist/plugin.py:252
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/xdist/plugin.py:252
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/xdist/plugin.py:252
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/xdist/plugin.py:252
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/xdist/plugin.py:252: DeprecationWarning: The --rsyncdir command line argument and rsyncdirs config variable are deprecated.
  The rsync feature will be removed in pytest-xdist 4.0.
    config.issue_config_time_warning(warning, 2)

<frozen importlib._bootstrap>:[48](https://github.com/googleapis/gapic-generator-python/actions/runs/6984468065/job/19007340405?pr=1873#step:7:49)8
<frozen importlib._bootstrap>:488
<frozen importlib._bootstrap>:488
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: Type google._upb._message.MessageMapContainer uses PyType_Spec with a metaclass that has custom tp_new. This is deprecated and will no longer be allowed in Python 3.14.

<frozen importlib._bootstrap>:488
<frozen importlib._bootstrap>:488
<frozen importlib._bootstrap>:488
<frozen importlib._bootstrap>:488
  <frozen importlib._bootstrap>:488: DeprecationWarning: Type google._upb._message.ScalarMapContainer uses PyType_Spec with a metaclass that has custom tp_new. This is deprecated and will no longer be allowed in Python 3.14.

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/proto/datetime_helpers.py:24
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/proto/datetime_helpers.py:24
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/proto/datetime_helpers.py:24
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/proto/datetime_helpers.py:24
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/proto/datetime_helpers.py:24: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    _UTC_EPOCH = datetime.datetime.utcfromtimestamp(0).replace(tzinfo=datetime.timezone.utc)

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:18
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:18
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:18
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:18
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:18: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:28[68](https://github.com/googleapis/gapic-generator-python/actions/runs/6984468065/job/19007340405?pr=1873#step:7:69): 12 warnings
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('google')`.
  Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
    declare_namespace(pkg)

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2[86](https://github.com/googleapis/gapic-generator-python/actions/runs/6984468065/job/19007340405?pr=1873#step:7:87)8
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('google.logging')`.
  Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
    declare_namespace(pkg)

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2348: 12 warnings
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2348: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('google')`.
  Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
    declare_namespace(parent)

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/pkg_resources/__init__.py:2868: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('google.iam')`.
  Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
    declare_namespace(pkg)

../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:20
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:20
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:20
../../home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:20
  /home/runner/work/gapic-generator-python/gapic-generator-python/.nox/showcase_unit_alternative_templates_mixins-3-12/lib/python3.12/site-packages/google/rpc/__init__.py:20: DeprecationWarning: Deprecated call to `pkg_resources.declare_namespace('google.rpc')`.
  Implementing implicit namespace packages (as specified in PEP 420) is preferred to `pkg_resources.declare_namespace`. See https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-namespace-packages
    pkg_resources.declare_namespace(__name__)

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.