feat: add support for python 3.10

[parthea]

No description provided.

[busunkim96]

E   AttributeError: module 'collections' has no attribute 'MutableMapping'
_ ERROR collecting .nox/unit-3-10-oauth2client-oauth2client-4-5dev/tmp/tests/test_schema.py _
tests/test_schema.py:22: in <module>
    import unittest2 as unittest
../lib/python3.10/site-packages/unittest2/__init__.py:40: in <module>
    from unittest2.collector import collector
../lib/python3.10/site-packages/unittest2/collector.py:3: in <module>
    from unittest2.loader import defaultTestLoader
../lib/python3.10/site-packages/unittest2/loader.py:13: in <module>
    from unittest2 import case, suite, util
../lib/python3.10/site-packages/unittest2/case.py:18: in <module>
    from unittest2 import result
../lib/python3.10/site-packages/unittest2/result.py:10: in <module>
    from unittest2.compatibility import wraps
../lib/python3.10/site-packages/unittest2/compatibility.py:143: in <module>
    class ChainMap(collections.MutableMapping):
E   AttributeError: module 'collections' has no attribute 'MutableMapping'

If unittest is the issue, we could move over to pytest for consistency with the other repos.

[tseaver]

From the PyPI page for unittest2:

unittest2 is a backport of the new features added to the unittest testing framework in Python 2.7 and onwards. It is tested to run on Python 2.6, 2.7, 3.2, 3.3, 3.4 and pypy.

We can just replace it with the stdlib's unittest: see #1564.