Check Enum definition for invalid base classes

[sobolevn]

Checks in Python's source:

• No classes after Enum base: https://github.com/python/cpython/blob/d05a66339b5e07d72d96e4c30a34cc3821bb61a2/Lib/enum.py#L971-L974
• Only a single data type: https://github.com/python/cpython/blob/d05a66339b5e07d72d96e4c30a34cc3821bb61a2/Lib/enum.py#L1011-L1012

Closes #11948

[97littleleaf11]

Something off-topic: the checker.py is somehow quite large, do you have any idea on split it into several submodules? cc @JukkaL

[sobolevn]

There are quite a lot of stateless methods that we can extract like new check_enum_bases. We only need fail callback there 🤔

[JukkaL]

Thanks, looks good! Sorry for the slow response, I'm slowly getting back to mypy code reviews.

[JukkaL]

We can split checker.py by defining a trait/interface that contains commonly used helper methods such as fail and passing the checker instance to another module using this new trait type to avoid an import cycle. We've already split semanal.py in this way (using SemanticAnalyzerInterface), though it's still very big.

The key would be to identify groups of related functionality that make sense to be moved away to a new module. Just moving random bits and pieces to another module doesn't improve the situation much. The semanal_foo modules can perhaps act as some inspiration.