Use specific EnumSelectField for PEP-435 enum columns

[mx-moth]

Enum columns are excellent, supporting them natively in the forms makes them very nice to work with.

[mx-moth]

This one requires a more recent sqlalchemy version, which is not explicitly mentioned in the tests. It works with more recent versions. What are your thoughts on dropping support for very old sqlalchemy versions?

[mlenzen]

Yeah, I'm good dropping support for very old versions, the last 0.9.x release was in 2015. Do you know what version of SQLAlchemy is required for this PR? I was also thinking Python 2 support should be dropped. Maybe I'll make one last release that supports old Python and old SQLAlchemy

[mx-moth]

sqlalchemy>=1.1 is required, as that is when the Enum column class first supported Python native Enums. The code as written also requires Python 3.

I could modify the code such that it supports sqlalchemy < 1.1, by doing something like if getattr(column, 'enum_class', None) is not None: instead of if column.enum_class:? Sadly I can't test based on the sqlalchemy version easily, as sqlalchemy does not expose the version information in a convenient tuple form, only as a string.

[mx-moth]

Superseded by #13, which includes tests, an EnumSelectMultipleField, and no merge conflicts!