math/cantor: Support ptyhon3.9

• instead of adding 'python3.9' to the list, simply patch in python-${PYTHON_VER} -- this way, this should fix two issues
  1. make it work with python
  2. help cmake find the "wanted" python version in unclean environments

PR: 250884
Reported by: Oleg Sidorkin <osidorkin@gmail.com>, fluffy (previously)