Improve some smaller typing issues - #26407

These can be set to `None` in their `__init__` or the property setter.

It's only there for `__init__` and immediately changed right after.

Mostly because these might be `TransformedBbox`.

These are semi-public.

The resample function from `_image` was missing, and bbox is allowed to
be a function like `get_window_extent`.

Most calls use a single type.

Internally, a `PillowWriter` is sometimes used, and this should be
supported from user code as well.

It is only ever created in `GridSpecBase.__getitem__` with `self` as
argument, so must accept those instead of `GridSpec`.

Instead, fall back to the `object.__init_subclass__` definition.

This change is not strictly correct, as these style classes don't
actually inherit from the surrounding class, but it allows one to pass
an instance of the style classes directly.