Add CI job for mypy

[davidar]

I've fixed up the remaining mypy errors outside of:

• mathics.builtin and mathics.eval: these still have quite a few errors, but are going to be a bigger job to go through than the core modules, and are probably easier to just knock off one by one incrementally
• test/ which I'd say are lower priority for type checking
• mathics/benchmark.py: is this still used? It just crashes when I try to run it

For now I've just set the mypy config to ignore these directories so we can run a CI job to ensure that the other core modules continue to typecheck, but might be better to be a bit more selective.

There were also a couple bits of code I've commented out as they appeared to be dead as best I could tell, but tell me if I'm mistaken.

[mmatera]

@davidar, let's merge this when you feel it is ready