State support for python 3.10, set minimal version of SciPy to 1.7

[hakonanes]

Description of the change

• State support for Python 3.10, closing Python 3.10 support #490. Test suite is now tested on Python 3.9 and 3.10 instead of 3.8 and 3.9, with minimal versions still tested on 3.7.
• Set minimal version of SciPy to 1.7, affecting but not closing Some refinement tests fail with the Powell algorithm with scipy 1.5.3 #500.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• [n/a] 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.
• New contributors are added to .all-contributorsrc and the table is regenerated.