This PR adds/enables support for Python version 3.10 and updates various CI/testing/build configuration files accordingly.

In addition, this PR refactors the project tox.ini file to (optionally) read and set the basepython configuration value from a TOX_BASEPYTHON environment variable, making it easier to test/build the application with different interpreters and/or on systems that don't have python3.10 installed (for example, to build the PyInstaller bundle with Python 3.9, one can invoke TOX_BASEPYTHON=python3.9 tox -e pyinstaller)