Derive EnumMeta from ABCMeta

[elazarg]

Ad-hoc fix python/mypy#2824, avoiding #1595.

Quoting the comment:

This is a temporary workaround to allow multiple creation of enums with builtins such as str as mixins, which due to the handling of ABCs of builtin types, cause spurious inconsistent metaclass structure.

This workaround is relatively harmless since enums are never subclassed further. It "adds" an incorrect register method to enums and Enum, and may cause incorrect isinstance checks, but I think it will be much less common and therefore less serious than the current problems.

[JelleZijlstra]

Thanks for diagnosing the issue and providing a workaround!