Type hints for 'return self'

[e10e3]

This is an attempt at #1417.
Some commits have already been done about this, but a few uses were left in the code.

I added type hints for the learn_one, learn_many, update, and revert methods in the base classes, meaning MyPy can now be used to detect child classes with the wrong return type.

I believe the only update method with a non-None return is the one in LazySearch, I think the boolean is voluntary and has a use case.

Since the original discussion was about update-like methods in general, I also updated a few methods like _fit where it didn't break the logic.

[e10e3]

I saw that SortedWindow.append has a return self as well, but I ultimately decided against changing it in fear of breaking something, even though it could fit the mould.
Please tell me if it should be changed as well.

[MaxHalford]

I saw that SortedWindow.append has a return self as well, but I ultimately decided against changing it in fear of breaking something, even though it could fit the mould.

You can change it. In theory it shouldn't return self. We can fix downstream issues if they arise.