Update CI

[rffontenelle]

• Update actions used. In particular, setup-python<4 uses node.js 12, which is deprecated.
• Python 3.10 is released, so moving it from -dev to release.
• Adds 'fail-fast: false' allows all Python versions set and flake8 and mypy run, instead of stopping the rest when one fails
• Add 'six' as dependency for test environment to satisfy tests/path.py.

P.S.: the workflow is failing due to changes from 6b3913f, as can be seen in tox logs.