Thanks, the fix is fine and there is no big need to backport it. For the record, I only came across this playing with runpy. Old message:

$ python3 -m .submodule
/sbin/python3: Error while finding spec for '.submodule' (<class 'ValueError'>: '.submodule' is not a relative name (no leading dot))

New message:

$ ./python -m .submodule
/media/disk/home/proj/python/cpython/python: Error while finding spec for '.submodule' (<class 'ValueError'>: no package specified for '.submodule' (required for relative module names))