Refactor imports from collections.abc, typing and typing_extensions for Python 3.9

[chopan050]

Overview: What does this pull request change?

Refactored imports from collections.abc, typing and typing_extensions. This PR is valid as long as we support Python 3.9. Changes might need to be done in the future after dropping support for that version.

Motivation and Explanation: Why and how do your changes improve the library?

• typing.Callable is a deprecated alias for collections.abc.Callable, which is why I used the latter instead.
• There were some imports of Any from typing_extensions when it's perfectly possible to import it from typing (unless we used Any as a base class, which we don't), so I did the latter instead.
• Sometimes, import typing was used instead of from typing import X, Y, Z. Sometimes, both kinds of imports were present in a file at the same time, or there were multiple from typing import X, Y, Z lines in different parts of the code, so I cleaned them.
• There was an import collections line in order to use collections.abc.Hashable and collections.abc.Iterable. In the same file, there was an import typing to use typing.Callable and other things, so I replaced the previous lines with from collections.abc import Callable, Hashable, Iterable.
• There was an import collections line in order to use collections.defaultdict, but other files simply used from collections import defaultdict, so I used the latter for consistency.

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested