Add typing-extensions as a runtime dependency

[pietermarsman]

Bug report

In #661 typing annotations were introduced. For certain functionalities the backport library of typing annotations, typing-extensions, is used. However, this dependency is not installed in the setup.py.

It is installed when the dev extras are installed because it is a dependency of mypy. However, regular users don't install that.

Another issue is that the cicd pipeline did not catch this. Because all of the tests are run with the dev extras installed. Some tests should run without the dev extras.