inline annotations for datastructures

[davidism]

Removed the stub files, and redid the annotations inline. Addressed some mypy findings this affected in other places. As I was going through everything I did a bit of refactoring and took some notes as well:

• I didn't keep track, but I'm pretty sure there were some inaccuracies in the stubs (and there probably still are in the new annotations), so users might notice some different messages from their type checker.
• Use super().method(...) instead of BaseClass.method(self, ...). There are some places where the non-super pattern is still needed.
• Call super().__init__() in some subclasses that were missing it.
• iter_multi_items and __ini__/update/etc methods accept Mapping, Collection, and Iterable instead of more specific types like dict, list, and tuple.
• keys()/values()/items() return Iterable instead of KeysView/etc, which doesn't match the Mapping interface. I couldn't think of a general way to efficiently implement these for the structures, and no one has ever brought it up, so I'm leaving it.
• Headers inherits from MutableMapping instead of nothing. This was true in the stubs, but wasn't actually reflected in the code.
• FileMultiDict.add_file accepts an os.PathLike path as the first argument (instead of only a str path).
• __getitem__/get/pop/etc only specify overrides for parameters that make sense. I couldn't figure out how to specify a complete set of overrides, and it didn't seem necessary anyway.
• Headers.getlist was be refactored to not check type is not None every iteration.
• Used positional-only args for things like Mapping.update(arg, /, **kwargs) and Headers.set(key, value, /, **kwargs) rather than taking and checking *args.
• Remove the internal _get_mode parameter from Headers.__getitem__, refactor various methods that used self[key] to work without needing that weird implementation.
• CombinedMultiDict.to_dict was an exact copy of the super implementation.
• None of these changes resulted in any tests breaking.

fixes #2970

[davidism]

Going to roll back accepting Collection for iter_multi_items and __init__, back to (list, tuple) instead. It was also matching str, bytes, and other types that should be treated as single values. See #2994