devel/py-mypy: update to 0.501

• Remove unneeded patches (superseded by upstream)
• Track minimum depedency on py-typed-ast

Change Log:

• typed_ast is used as the default parser (i.e. --fast-parser is now the default)
• Python 3.6 features supported by default
• Simples metaclass support added
• The --warn-no-return flag is now default: warns about missing return statements
• A function that doesn't return can be annotated (using mypy_extensions.NoReturn)
• Redundant Generic[T] no longer needed
• Checking of assignments in a class body
• The --strict flag is added, that turns on most additional checks

PR: 218303
Approved by: Florian Limberger <flo@snakeoilproductions.net>