python,python3: move shebang handle in install script

python,python3: move shebang handle in install script

This extends the Python[3] shebang fixup to all packages.
Only Python scripts in `/usr/bin` will be handled at the moment. Later it
may make sense to also cover executables in `/bin`, though typically Python
executables shouldn't be placed there.

Previously the shebang handling was only done for python[3]-pip &
python[3]-setuptools.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>