Return exact type from freeze_time() context manager

[mtvx]

with freeze_time() as factory:

factory can be either StepTickTimeFactory or FrozenDateTimeFactory. It's a bit annoying, because the two have slightly different APIs (e.g. tick() takes different types).

Could this one be more type friendly (I don't have suggestion... but somehow). Now, in order to satisfy mypy, one has the cast the factory into specific type, assert it, or smth?