Fix various small type hint issues by QuLogic · Pull Request #31920 · matplotlib/matplotlib

The `_XYPair` is initialized with two lists, and is treated as such
(calling `.append`, etc.)

The `_ColorbarSpine` is a subclass of `mspines.Spine`, not
`mspines.Spines`.

This was missing a few parameters.

This is a mutable version of `BboxBase`, and all its properties should
be writable. Due to a deficiency in mypy [1], we need to ignore some
attribute access type errors.

python/mypy#5936

Axes should definitely be able to be placed in SubFigures, and that's
the accepted type for `_AxesBase.set_figure`.

This mostly means the paths are passed directly to `open` or similar.

No need for `Union` any more.