Remove support for Python 3.7

[hakonanes]

Description of the change

Drops support for Python 3.7. Closes #482.

The changes I've made I did after searching for "3.7" in the source code. I didn't search very long, but I could not find any code which could be removed or simplified.

Progress of the PR

• [n/a] Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

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 orix/__init__.py and in
  .zenodo.json.