The edx-platform codebase already includes quite a few type annotations, but
they were not regularly checked. This may cause problems, when the annotations
themselves include errors (as we found out in some of the learning_sequences
annotations). So here, we add mypy as a dev requirement and introduce a make
command to run mypy regularly. Mypy runs on a very small portion of the total
edx-platform, as configured in mypy.ini. Our hope is that developers will add
more and more modules to this configuration file, until we can eventually run
mypy on the full code base.

See discussion: https://discuss.openedx.org/t/dev-notes-running-mypy-on-edx-platform/4860