pysetup.mk: Run commands on multiple `python's, possibly in parallel.

Most targets delegated to `setup.py' are now run, in parallel, on
multiple Python versions listed in the new Makefile variable `PYTHONS'.
The exceptions are `gen' and `dist', which only make sense with a single
Python.

Targets which can run multiple Python versions now support per-version
options for targets: set the variable OPTS-TARGET/PYTHON (e.g.,
`OPTS-install/python3.5').