Remove versioneer from requirements.txt

[maresb]

Context: Versioneer was added in #6078, installed in "vendored mode" so that the necessary code is installed into versioneer.py.

It seems strange to me that versioneer is an install requirement. It looks like nothing breaks when I remove it.

In order to enable CI, I had to add requirements.txt to the list of test-triggering files in .github/workflows/tests.yml.

What is this PR about?
...

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• ...

Bugfixes / New features

• ...

Docs / Maintenance

• Remove (apparently) unnecessary versioneer requirement
• Trigger CI on a PR which modifies requirements.txt

[codecov]

Codecov Report

Merging #6135 (e4ce1d4) into main (5236d3e) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6135   +/-   ##
=======================================
  Coverage   92.90%   92.90%           
=======================================
  Files         101      101           
  Lines       20945    20945           
=======================================
  Hits        19459    19459           
  Misses       1486     1486           

[twiecki]

Thanks!