Typing cleanup

[lemon24]

Clean typing stuff. Might be able to use pyupgrade for this.

To do:

• [x] from __future__ import annotations (and the changes it enables)
• [ ] don't depend on typing_extensions at runtime (example)
• [x] typing.Self (supported by mypy master, but not by 0.991)
• [x] show types for data objects in docs
• [x] ~~move TagFilter, {Feed,Entry}Filter options to _storage (todo)~~ no, used by _search as well, just remove todo
• [ ] make DEFAULT_RESERVED_NAME_SCHEME public
• [ ] move reader.core.*Hook to types

To not do:

• Consolidate (public) aliases under reader.typing (Flask does it)?
  • No, reader.types is likely good enough (but maybe consolidate them in one place in the file).
• import typing as t, import typing_extensions as te (Flask does it)?
  • Fewer imports, but makes code look kinda ugly.
  • Also,

    When adding types, the convention is to import types using the form from typing import Union (as opposed to doing just import typing or import typing as t or from typing import *).