Support native python enum generation

[sheinbergon]

Changes

Fixes #408

Adds Python enum generation for database enum types

Native database ENUMs

• Generates Python enum classes from native ENUM types
• Handles named/unnamed enums with automatic deduplication
• Use --options nonativeenums to disable

Regarding Synthetic ENUMs for VARCHAR columns with CHECK Constraints

• Retain existing capability of detecting CHECK (column IN ('val1', 'val2', ...)) patterns and converts them to Python enums.
• Preserves original CHECK constraint for database validation
• Use --options nosyntheticenums to disable

Generated Code

class UsersStatus(str, enum.Enum):
    ACTIVE = 'active'
    INACTIVE = 'inactive'
    PENDING = 'pending'

status: Mapped[UsersStatus] = mapped_column(Enum(UsersStatus), nullable=False)

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: 97.459% (+0.1%) from 97.36%
when pulling c26f283 on sheinbergon:python-native-enums
into 638861c 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

[sheinbergon]

@agronholm can you have a look now?

[agronholm]

I'll review this tomorrow.

[sheinbergon]

@agronholm did you have a chance to review this PR?

[agronholm]

Doing that now.

[agronholm]

Some change requests, questions and comments.

[agronholm]

Hopefully this is the last round.