Type error when using an ABC as a spec

[mcous]

overview

Creating doubles of abstract interfaces defined using ABC is a great way to use to use decoy, and it works! However, given the way Decoy::create_decoy is typed, mypy throws an error when you try to pass an ABC as spec.

acceptance criteria

• Passing an ABC as spec does not throw a type error

implementation notes

• I think a mypy plugin could work here, but that feels like a last resort
• Currently, for creating class doubles, we rely on Type[GenericT]
  • However, mypy does not allow Type to wrap abstract classes
  • See Use case for typing.Type with abstract types python/mypy#4717

[mcous]

Closed by #20