chore: remove use of old external "mock" module

[a-detiste]

mock is now part of the Python standard library, available as unittest.mock in Python 3.3 onwards.

https://github.com/testing-cabal/mock

[musicinmybrain]

See also #1361, #1055.

[chalmerlowe]

Made multiple updates to account for:

• incorrectly spelled library names
• adjust the order of certain imports to align with flake8 guidelines for imports

[chalmerlowe]

@a-detiste Thanks for lining this up. You have been patient to see it merged.
I noticed that there appear to be several places where we are missing a transition from mock to unittest. Are you willing to update the PR?

If yes, please address the following errors:

_____________ ERROR collecting tests/compute_engine/test__mtls.py ______________
ImportError while importing test module '/tmpfs/src/github/google-auth-library-python/tests/compute_engine/test__mtls.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tests/compute_engine/test__mtls.py:20: in 
    import mock
E   ModuleNotFoundError: No module named 'mock'
_____________ ERROR collecting tests/test_agent_identity_utils.py ______________
ImportError while importing test module '/tmpfs/src/github/google-auth-library-python/tests/test_agent_identity_utils.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/local/lib/python3.14/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tests/test_agent_identity_utils.py:21: in 
    import mock
E   ModuleNotFoundError: No module named 'mock'

Also, if you would be willing to include me as a maintainer on your branch just in case there is an additional issue, I can simply make the mods to the PR directly and avoid the back and forth.

Any help you can offer is appreciated. If your schedule does not allow you to continue supporting, that is understandable and I can see what I can do.