refactor: Use Parameter class directly in type hints instead of _Parameter protocol

[binste]

Turns out I was wrong in #3208 (comment) which is nice in this case :) We can use the Parameter class directly in type hints without running into circular imports thanks to typing.TYPE_CHECKING. See https://mypy.readthedocs.io/en/stable/runtime_troubles.html#import-cycles for more details but in summary, type checkers can be much more flexible how they import stuff as they don't run actual code so they can also deal with circular imports.

There is no impact for users except that instead of _Parameter as type hints:

they now see the less confusing Parameter:

[dangotbanned]

Fixed in: https://github.com/dangotbanned/altair/blob/3ec3e6abfeb742942d1b2bc5d1a0bd4c4c5fa7d2/altair/vegalite/v5/schema/mixins.py#L448

[mattijn]

Is this PR still applicable after merging #3430?

[dangotbanned]

Is this PR still applicable after merging #3430?

@mattijn I think all of this was covered in that PR, which I could have conveyed better in #3405 (comment)

[binste]

Good point! I'll remove some comments which are now outdated in #3405