Add typing-extensions as dependency and pin mypy version

[nicarl]

What does this changes

The PR adds typing-exntesions as a dependency in setup.py and pins the version of mypy to 0.910.

What was wrong

@obieler raised the issue that from typing_extensions import Literal does not work on python 3.6 and 3.7 without installing typing-extensions (#1536).
Furthermore the mypy version was not pinned. It is advised to pin the mypy version to avoid that new typing errors appear due to changes in mypy, which would make the CI and errors harder to reproduce (https://sethmlarson.dev/blog/2021-10-18/tests-arent-enough-case-study-after-adding-types-to-urllib3).

How this fixes it

typing-extensions is added as a dependency and solves this issue.

[fcurella]

Fixes #1556

[fcurella]

In install_requires, I'd rather require just typing-extensions than the whole mypy.

Q: if we do that, does it make the --install-types option in mypy --install-types redundant?

[nicarl]

In install_requires, I'd rather require just typing-extensions than the whole mypy.

Q: if we do that, does it make the --install-types option in mypy --install-types redundant?

Valid point concerning the dependency, better to add just typing-extensions.
We still require the --install-types flag, as some libraries do not contain types themselves but stubs exists. They need to be installed from https://github.com/python/typeshed. For our case python-dateutil has only stubs which need to be installed.

(details can be found here: https://mypy.readthedocs.io/en/stable/installed_packages.html#using-installed-packages)

[fcurella]

Thank you!