Migrate to new type aliases

[bswck]

PEP 585 combined with PEP 563 allows to use types directly (not their typing runtime aliases).
So, for example:

• let's not import Callable from typing, but from collections.abc instead
• let's not import typing.Type, but use the built-in type instead

etc., since

1. we are using __future__.annotations
2. we're not evaluating generic type annotations such as Iterable[...] at runtime (breaks compatibility with Python 3.8).

PEP 604 combined with PEP 563 allows to drop typing.Union, although not deprecated.