Remove Python 3.7 Support

[CSSFrancis]

Description of the change

Remove python 3.7 support

Progress of the PR

• Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black

Minimal example of the bug fix or new feature

>>> from diffsims.utils import sim_utils
>>> # Your new feature...

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in CHANGELOG.rst.
• Contributor(s) are listed correctly in credits in diffsims/release_info.py and
  in .zenodo.json.

[hakonanes]

I took the liberty of updating our build actions versions.