Add official support for Python 3.10 and 3.11

[browniebroke]

The main blocker were the test runner, nose which is no longer maintained and has several open issues for recent versions of Python:

• https://github.com/nose-devs/nose/issues/1099
• https://github.com/nose-devs/nose/issues/1118
• https://github.com/nose-devs/nose/issues/1116
• https://github.com/nose-devs/nose/issues/1122

I've replaced nose test runner by pytest (which is widely used in the Python community), let me know if you disagree with this choice, and if you prefer another alternative.

Related: #265