Type checking with mypy

[carmenbianca]

I'd been meaning to do this for a while. We have all these type annotations, but don't check whether they're valid.

[carmenbianca]

Almost completed, except for report.py and lint.py. I'll wait on #654 to merge before doing that.

[carmenbianca]

Not almost completed; I disallowed undefined and incomplete function defs, and will need to do some work on that.

[nicorikken]

My approval for the current changes, I reviewed all code changes. Thanks, great work!
Besides adding typing, I noticed a couple of improvements in logic, variables and handling like handling copyright statements. In the future perhaps specific types can be used for copyright statements and years as not to rely on str. I learned a couple of new constructs.

[carmenbianca]

Rebased this on top of #759