test_iam_credentials.py imports mock, but this fails on Python 3.10

[major]

The following errors appears when running the tests on Python 3.10:

+ /usr/bin/pytest tests
============================= test session starts ==============================
platform linux -- Python 3.10.0b4, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /builddir/build/BUILD/python-iam-2.3.0
collected 0 items / 1 error

==================================== ERRORS ====================================
___ ERROR collecting tests/unit/gapic/credentials_v1/test_iam_credentials.py ___
ImportError while importing test module '/builddir/build/BUILD/python-iam-2.3.0/tests/unit/gapic/credentials_v1/test_iam_credentials.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib64/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/gapic/credentials_v1/test_iam_credentials.py:17: in <module>
    import mock
E   ModuleNotFoundError: No module named 'mock'
=========================== short test summary info ============================
ERROR tests/unit/gapic/credentials_v1/test_iam_credentials.py
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.13s ===============================

[busunkim96]

It looks like this issue eventually resulted in googleapis/gapic-generator-python#957. mock must be installed separately for mock.Asyncmock.

Test dependencies are listed in the noxfile.py, but this list is not accessible to folks running the tests outside of nox. I looked into tests_require, but https://setuptools.pypa.io/en/latest/userguide/dependency_management.html?highlight=tests_require#optional-dependencies states:

tests_require is now deprecated

There might be another good way to share the test dependencies (a separate requirements.txt?) but I think it will require a bit more thought.

I am going to close this issue since the unit tests are runnable in 3.10 as long as mock is installed.