Fix typing for MyPy

[rodrigogiraoserrao]

With the API more stable, we should get Textual passing Mypy.

There are currently 148 errors, but many of them have the same root cause.

It may require a few passes to get all of them. Suggest spending a day on fixing typing, and we will review the situation after.

[rodrigogiraoserrao]

Follow-up: see if my suggestion in #1813 breaks typing.

[rodrigogiraoserrao]

Edit: By the time I picked this up, there were 85 errors.

@darrenburns two issues read:

src/textual/_two_way_dict.py:41: error: Incompatible return value type (got "Optional[Value]", expected "Value")  [return-value]
src/textual/_two_way_dict.py:52: error: Incompatible return value type (got "Optional[Key]", expected "Key")  [return-value]

To guide my fix, I need to know if the rationale here is that the methods .get and .get_key never fail?
Or are they supposed to be mirroring dict.get? In which case, we probably need to restore the default: T | None = None annotation and fix typing in the correct way.

[rodrigogiraoserrao]

These have been axed down to around 30 in Python 3.7.
Most of them come from the widget Data Table and interactions with the TwoWayDict implemented in _two_way_dict. The PR above contains a possible approach but this needs to be validated with @darrenburns.

[willmcgugan]

Typing is an ongoing thing. We will have another pass soon.

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