AttributeError: type object 'TestCase' has no attribute 'failUnless' with Python 3.12

[sebastic]

Context

The testsuite fails with Python 3.12:

============================= test session starts ==============================
platform linux -- Python 3.12.1, pytest-7.4.3, pluggy-1.3.0
rootdir: /build/mapproxy-2.0.0+dfsg/.pybuild/cpython3_3.12_mapproxy/build
configfile: pytest.ini
collected 920 items / 82 errors

==================================== ERRORS ====================================
___________________ ERROR collecting mapproxy/test/helper.py ___________________
/usr/lib/python3/dist-packages/_pytest/runner.py:341: in from_call
    result: Optional[TResult] = func()
/usr/lib/python3/dist-packages/_pytest/runner.py:372: in <lambda>
    call = CallInfo.from_call(lambda: list(collector.collect()), "collect")
/usr/lib/python3/dist-packages/_pytest/doctest.py:567: in collect
    module = import_path(
/usr/lib/python3/dist-packages/_pytest/pathlib.py:567: in import_path
    importlib.import_module(module_name)
/usr/lib/python3.12/importlib/__init__.py:90: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1387: in _gcd_import
    ???
<frozen importlib._bootstrap>:1360: in _find_and_load
    ???
<frozen importlib._bootstrap>:1331: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:935: in _load_unlocked
    ???
<frozen importlib._bootstrap_external>:994: in exec_module
    ???
<frozen importlib._bootstrap>:488: in _call_with_frames_removed
    ???
mapproxy/test/helper.py:26: in <module>
    from mapproxy.test import mocker
mapproxy/test/mocker.py:126: in <module>
    class MockerTestCase(unittest.TestCase):
mapproxy/test/mocker.py:456: in MockerTestCase
    assertTrue = unittest.TestCase.failUnless
E   AttributeError: type object 'TestCase' has no attribute 'failUnless'
___________________ ERROR collecting mapproxy/test/mocker.py ___________________
[...]

The alias was removed, see: https://docs.python.org/3/whatsnew/3.12.html#unittest-testcase-removed-aliases

Expected Behavior

No test failure

Actual Behavior

Test failure

Possible Fix

Use assertTrue().

Steps to Reproduce

1. Run tests with Python 3.12

Context

Build the Debian package with Python 3.11 and Python 3.12.

Your Environment

• Version used: 2.0.0
• Environment name and version (e.g. Python 2.7.5 with mod_wsgi 4.5.9): Python 3.12
• Server type and version: N/A
• Operating System and version: Debian unstable

[ramunasd]

I see tests pipeline is not enabled for python 3.12.
Can You submit PR with the fix and enabled python 3.12? @sebastic