Python 3.11 support

[corranwebster]

Run pip-based tests on 3.11. We can't do Wx, PySide2 or PyQt5 easily since there are no wheels.

[corranwebster]

To proceed with this we need either:

• a new celiagg release
• install from celiagg main

[corranwebster]

Tests should pass once https://github.com/celiagg/celiagg/pull/118 is merged

[corranwebster]

Thanks!