Support native python enum generation

[sheinbergon]

Changes

Fixes #408

Support native python enum generation for models of all kins

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've added a new changelog entry (in CHANGES.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/sqlacodegen/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[coveralls]

coverage: 97.444% (+0.08%) from 97.36%
when pulling 75d603e on sheinbergon:python-native-enums
into 90831a7 on agronholm:master.

[agronholm]

Looks like there's a coverage drop. Can that be resolved?

[sheinbergon]

Looks like there's a coverage drop. Can that be resolved?

Ah, probably after my recent amendments to this PR. Sure, I'll fix them and ping you