Overrides

Fix https://github.com/mkorpela/overrides/issues/65

Fix case where type is defined as string.

Improved support for mypy types and type checking with py.typed.
Distribution as a wheel also.

EnforceOverrides now checks argument and return value signatures for overriding problems.
This is a backwards incompatible change.
Special thanks to ashwin153 for all the hard work on making this possible!

https://github.com/mkorpela/overrides/issues/35