Upgrade minimum version of google-cloud-core to 1.4.4

[parthea]

There is a dependency on google-cloud-core>=1.4.1 in setup.py. We should bump the minimum version to 1.4.4. In older versions of google-cloud-core, six was used in the code but was not explicitly listed as a dependency resulting in runtime errors in environments where it is not already installed.

https://github.com/googleapis/python-cloud-core/releases/tag/v1.4.4

python-spanner/setup.py

Lines 38 to 40 in 498dba2

	dependencies = [
	"google-api-core[grpc] >= 1.34.0, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.*,!=2.4.*,!=2.5.*,!=2.6.*,!=2.7.*,!=2.8.*,!=2.9.*,!=2.10.*",
	"google-cloud-core >= 1.4.1, < 3.0dev",

This is causing the unit-3.7 continuous build to fail. See build log here

nox > Running session unit-3.7
nox > Creating virtual environment (virtualenv) using python3.7 in .nox/unit-3-7
nox > python -m pip install mock asyncmock pytest pytest-cov pytest-asyncio -c /tmpfs/src/github/python-spanner/testing/constraints-3.7.txt
nox > python -m pip install -e . -c /tmpfs/src/github/python-spanner/testing/constraints-3.7.txt
nox > py.test --quiet --cov=google.cloud.spanner --cov=google.cloud --cov=tests.unit --cov-append --cov-config=.coveragerc --cov-report= --cov-fail-under=0 tests/unit
/tmpfs/src/github/python-spanner/.nox/unit-3-7/lib/python3.7/site-packages/coverage/inorout.py:507: CoverageWarning: Module google.cloud.spanner was never imported. (module-not-imported)
  self.warn(f"Module {pkg} was never imported.", slug="module-not-imported")
/tmpfs/src/github/python-spanner/.nox/unit-3-7/lib/python3.7/site-packages/coverage/inorout.py:523: CoverageWarning: Module google.cloud was previously imported, but not measured (module-not-measured)
  self.warn(msg, slug="module-not-measured")

==================================== ERRORS ====================================
__________ ERROR collecting tests/unit/test__opentelemetry_tracing.py __________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test__opentelemetry_tracing.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test__opentelemetry_tracing.py:13: in <module>
    from google.cloud.spanner_v1 import _opentelemetry_tracing
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
__________________ ERROR collecting tests/unit/test_batch.py ___________________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test_batch.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_batch.py:18: in <module>
    from google.cloud.spanner_v1 import RequestOptions
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
_________________ ERROR collecting tests/unit/test_database.py _________________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test_database.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_database.py:25: in <module>
    from google.cloud.spanner_v1 import RequestOptions
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
_________________ ERROR collecting tests/unit/test_session.py __________________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test_session.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_session.py:17: in <module>
    from google.cloud.spanner_v1 import RequestOptions
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
_________________ ERROR collecting tests/unit/test_snapshot.py _________________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test_snapshot.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_snapshot.py:19: in <module>
    from google.cloud.spanner_v1 import RequestOptions
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
_________________ ERROR collecting tests/unit/test_spanner.py __________________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test_spanner.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_spanner.py:18: in <module>
    from google.cloud.spanner_v1 import (
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
_______________ ERROR collecting tests/unit/test_transaction.py ________________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/test_transaction.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_transaction.py:18: in <module>
    from google.cloud.spanner_v1 import RequestOptions
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'
________ ERROR collecting tests/unit/spanner_dbapi/test_parse_utils.py _________
ImportError while importing test module '/tmpfs/src/github/python-spanner/tests/unit/spanner_dbapi/test_parse_utils.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.7/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/spanner_dbapi/test_parse_utils.py:18: in <module>
    from google.cloud.spanner_v1 import param_types
google/cloud/spanner_v1/__init__.py:65: in <module>
    from google.cloud.spanner_v1.client import Client
google/cloud/spanner_v1/client.py:33: in <module>
    from google.cloud.client import ClientWithProject
.nox/unit-3-7/lib/python3.7/site-packages/google/cloud/client.py:21: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'