build-sys: use automake's PYTHON primary for __init__.py

Actually the initial reason for this commit was to remove execute
permission from installed __init__.py.

Now after discovering automake's _PYTHON suffix we slightly cleanup
Makemodule.am and we will install byte compiled .pyc and .pyo files.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>