Use Python 3.10 on CI

[mkniewallner]

Before submitting your pull-request, make sure the following is done.

• Fork the repository and create your branch from master so that it can be merged easily.
• Update changelogs/next.md with your change (include reference to the issue & this PR). Not applicable
• Make sure all of the significant new logic is covered by tests. Not applicable
• Make sure all quality checks are green (Gazr specification).

Learn more about contributing

• Use Python 3.10 to run style, now that it's been officially released.
• Run tests against multiple Python version on master branch
• Bump cibuildwheel to 2.1.2 to benefit from building wheels against Python 3.10.0rc2

Note: for now, jobs for uploading sdist/wheels still run against Python 3.9 because twine doesn't officially support Python 3.10 yet.