Fix issue 86 drop Python 3.6 build and add Python 3.11 build

[owenlamont]

This is some random fixes to the build actions and fixed a regression I introduced in a previous PR (didn't realise functools.cache wasn't introduced until Python 3.9). Changes include:

• Drop Python 3.6 setup and add Python 3.11 setup
• Replace functools.cache with the functionally equivalent older format functools.lru_cache(maxsize=None) to be compatible with Python 3.7 upwards
• Added Scipy to CI install but kept it as an optional dependency (I'm assuming it is intended to be an optional dependency)

[erikbern]

Thanks!

[owenlamont]

No problem, let me know if you do another release. I'm keen to use this package more.