Mypy improvements

[Mityuha]

Support latest mypy version (1.x.x). For example, methods that use override should be sorted in the right order. Also some things regarding coroutines' type hints may be improved as well

[Mityuha]

add annotations to __aenter__ method

    async def __aenter__(self: T) -> T:
        """Open up your real bakery."""
        return await type(self).aopen()

[Mityuha]

Also think about static analysis for non-existent attributes. Now there is no error if bakery hasn't asd attribute -- bakery.asd is the valid statement. That's all because of __getattribute__ bakery's method

[Mityuha]

The latest version of mypy also requires python >= 3.8. But bakery has the Python >= 3.6 support. Think about it