Makefile.python: use python3 -m nose as a test entry point

Summary:
Using python3 -m nose instead of the nosetests3 entrypoint allows
us to call the nosetests that depends of the python version we chose instead of
one that might not be up to date or present at all.

Test Plan: make check

Reviewers: Reviewers!, olasd, zack, ardumont

Differential Revision: https://forge.softwareheritage.org/D120