Protocols for AcceptanceCriterion and WeightScheme

[N-Wouda]

I am not a fan of acceptance criteria or weight schemes requiring inheritance to pass type checks. I would much rather use protocols (this PEP), but those are not available in Py3.7.

So we have to wait until EOL of 3.7, which will happen in July 2023.

[N-Wouda]

They're in typing-extensions for earlier Python versions. Might be worth adding this as a dependency, since many systems will likely already have it installed.

[N-Wouda]

The WeightScheme base class has since been renamed to OperatorSelectionScheme, but the point stands.

[N-Wouda]

There's nothing blocking this anymore, so I will try to get this set up in the coming weeks.

[N-Wouda]

Since the OperatorSelectionScheme base class has an implementation that does meaningful work, I will leave that explicit inheritance structure intact.