Update actions used in the workflows

[matrss]

Updates actions/checkout to v4 and actions/setup-python to v4. Some trailing whitespace was removed by my editor as well.

Purpose of PR?:

Fixes #2088

Does this PR introduce a breaking change?

No

[ReimarBauer]

@matrss I think that should better go into stable and later merged into develop. I don't know when V4 is required and how long we need until we release 9.0.0.

[matrss]

I am pretty sure that actions/checkout uses standard git commands to fetch the repository and v3 will therefore continue to work for a long time. I don't think we have to worry about this if we will release 9.0.0 in the next 10 years. If it ever does stop working we can still do a backport.