enable self type annotation

[tylerflex]

Before, when type annotating a class method, could not reference the class name. Instead, we put it in quotes:

class MyClass:
    def copy(self) -> 'MyClass':
        pass

In python 3.7 +, we can use the class name in the annotations by importing

from __future__ import annotations

class MyClass:
    def copy(self) -> MyClass:
        pass

note, breaks python 3.6. Are we even supporting it though?

[momchil-flex]

Sounds good, python >=3.7 is already in our requirements.