This commit adds support for static typechecking with mypy.
Notable additions:
- A new target "check" which does the type checking
- Py-mods.mk, meant to be included from a directory containing python modules
in subdirectories, but not being a python module itself. It makes the all
target depend on check only if PY_RUN_CHECK_AFTER_BUILD is defined and
true. That's because pypy is under heavy development, and the Ubuntu 18.04
version is too old to work for lots of the code.
Signed-off-by: Jan Lindemann <jan@janware.com>
