build: Update classifiers stating Python versions

[EliahKagan]

This updates the classifiers listed in setup.py.

Removals:

• Python 2 classifiers. Although a significant amount of the code remains written in the common subset of Python 2 and Python 3, the codebase overall no longer supports Python 2. For example, collections.abc is imported.
• Python 3.5. Not all tests reliably pass. Specifically, test_regression_208 sometimes passes and sometimes raises TypeError. Later versions do not show such nondeterministic behavior, even when the test is run many times in succession (it always passes).
• Python 3.6. Tests still pass. This version does appear still to work. However, 3.6 has been unsupported by the PSF for a significant time, and it is no longer tested in tox.ini or in the CI test workflow.

Additions: 3.8, 3.9, 3.10, and 3.11. (Along with 3.7, which was already listed, these are the actual supported versions. These versions are tested regularly on CI, with all tests passing.)

While editing the classifiers, I also removed some unused imports.

I was unsure if I should really remove the 3.6 classifier at this time, but removing it doesn't prevent installation on 3.6, removing it achieves consistency with changes in #240, and from #240 (comment) it appears that 3.6 support is not a goal.

It may make sense to add 3.12 later once any changes needed to install and pass all tests locally and on CI have been made, but I did not add it here.

I was unsure if the commit here should be prefixed build:, cleanup:, or something else. I can amend the message and force-push if that would be helpful.