python: Drop explicit version where possible

python: Drop explicit version where possible

Some of our scripts are known to work both with Python 2 and
Python 3, so for them we shouldn't be forcing any specific
version of the interpreter when they're called directly; we
always use $(PYTHON) explicitly in our build rules anyway.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>