makefile.python: hardening - drop virtualenv, drop build support py2

From py3.3 on a subset of virtualenv is built-in, so you can run '-m venv' ot of
the box.

- replace: $(PY_ENV_BIN)/pip --> $(PY_ENV_BIN)/python -m pip
- remove obsolete virtualenv-exe target and adjust VTENV_OPTS
- remove obsolete msg-pip-exe target
- print list of py launchers available from $(PY_ENV_BIN) to the log
- fix hard coded ./local

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>