REL: Python 3.11, CI: `pytest`

REL: support Python 3.10, 3.11
CI: add Python 3.10, 3.11 to test environments
CI: use GitHub Actions
TST: use pytest
API: do not call matplotlib.pyplot.show() from within a library function
REF: use shlex.split() in setup.py for invocations of git
TST: avoid test failure when matplotlib is not installed (optional dependency), by using unittest.skipIf()
and refactoring

Merging intended within days.

tulip-control / polytope