Add support for Python 3.10

[joshuadavidthomas]

• adds Python 3.10 as a version in the testing matrix for the PyTest GitHub Action
• removes 3.10-dev from the include section at end of testing matrix and provisionally adds 3.11-dev commented out
• adds py310 to the envlist in tox.ini
• updates the trove classifiers to add 3.10 as a supported option

Passing tests

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[joshuadavidthomas]

Here is the traceback from before I commented out 3.11-dev:

Traceback (most recent call last):
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/py/_vendored_packages/apipkg/__init__.py", line 145, in __makeattr
    modpath, attrname = self.__map__[name]
                        ~~~~~~~~~~~~^^^^^^
KeyError: '__spec__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/venv-3.11/bin/tox", line 5, in <module>
    from tox import cmdline
    ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/tox/__init__.py", line 32, in <module>
    from .session import cmdline  # isort:skip
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/tox/session/__init__.py", line 20, in <module>
    from tox import reporter
    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/tox/reporter.py", line 127, in <module>
    _INSTANCE = Reporter()
                ^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/tox/reporter.py", line 32, in __init__
    self._reset(**kwargs)
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/tox/reporter.py", line 38, in _reset
    self.tw = py.io.TerminalWriter()
              ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/py/_vendored_packages/apipkg/__init__.py", line 152, in __makeattr
    result = importobj(modpath, attrname)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/py/_vendored_packages/apipkg/__init__.py", line 72, in importobj
    module = __import__(modpath, None, None, ['__doc__'])
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1083, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1043, in _find_and_load_unlocked
  File "/home/runner/venv-3.11/lib/python3.11/site-packages/py/_vendored_packages/apipkg/__init__.py", line 150, in __makeattr
    raise AttributeError(name)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: __spec__
Error: Process completed with exit code 1.

I figured it would be outside the scope of this PR to fix this error, hence including the future version commented out.

[atugushev]

LGTM 👍 Thanks @joshuadavidthomas