Python 3 Metaclasses [Support ABC and Enums - Part 1]

[anivegesana]

This PR is part 1 of the broken down version of #450. It extends support of metaclass pickling to features added in Python 3.0, 3.6, and 3.7 (https://peps.python.org/pep-3115/, https://peps.python.org/pep-0487/, https://peps.python.org/pep-0560/).

[anivegesana]

I think the coverage issue can be ignored. The coverage passes in #450 and it is failing here because the ABC and enum tests are not in this PR.

[anivegesana]

@mmckerns Should I add test cases that test this feature without using enums or ABCs, or is this feature ready for review?

[mmckerns]

Please always request a review when you feel a PR is ready to be reviewed. Otherwise, it's hard to know what the state of the PR is. The general rule is that you should always add tests cases, unless the change is trivial.

[anivegesana]

I think that I don't have the permissions to request a review. I am just marking PRs that aren't ready for review as drafts. Will create test cases for these features to improve coverage.

[anivegesana]

@mmckerns It should be ready for review now.

[mmckerns]

lgtm

[anivegesana]

The size of this PR seems to be more manageable. I will make the remaining PRs roughly the same size. This PR merged 148/599 changed lines (additions/deletions), so we have merged 1 out of 4 PRs need for supporting ABCs and Enums. 🎉

This is sizable progress. @mmckerns, hope that this plan isn't too cumbersome.

[mmckerns]

note does not seem to close #250 [i.e. enum are not yet supported)