New unittest file consists of two tests. Both
run mypy, but one does so with arguments that are
expected to allow the test to pass. The other does
not provide those arguments and is expected to fail.
The expected failure is intentional, to serve as
an ongoing reminder to try to 1) move towards having
strict typing in the codebase and to 2) not
add any additional errors/warnings in the meantime.
Separated requirements into runtime (requirements.txt)
and additional development (requirements-dev.txt).
Modified devcontainer and workflow to reflect the
change.
