When running doctest on a module that performs a relative import in a PEP-420
namespace package, you get an ugly error saying
Parent module '' not loaded, cannot perform relative import
This program imports the module using importlib and runs doctest.testmod on
that to resolve this problem.
You may specify either a dot-separated module path or a filename relative to the current directory.
If you specify a filename like foo/bar/baz.py, this program will add the
current working directory to PYTHONPATH and import foo.bar.baz.
If instead you specify foo.bar.baz directly, PYTHONPATH is not modified.