First attempt at trying to have a clean mypy pass on our code base. This change
just adds support for "make typecheck", not used anywhere unless explicitly
invoked in specific module dirs.

With this conf it is already possible to cleanly typecheck swh.{model, core,
objstorage}, provided that some additional minor changes are committed in the
respective repos---changes that I also have in separate diffs, but that will
need to wait for Python >= 3.6, as they require PEP 526 variable annotations.

Interestingly enough, even if these are still mostly no-op mypy run, making
mypy pass made me find a couple of minor bugs (e.g., missing optional "import"
statements hidden by "# noqa" annotations that were added for unrelated
reasons).