python,python3: add Py[3]Shebang functions & move outside of script

python,python3: add Py[3]Shebang functions & move outside of script

Some packages just install some Python binaries, that may need their
shebang fixed.
This change adds some utilities to help with that and try to centralize the
sed rules a bit.

It also removes the logic from the `python-package-install.sh` into the
`python-package[3].mk` files. This does 2 things:
1. It minimizes the need for the shell script to know the Python
   version 2/3
2. Makes the logic re-usable in packages; especially if the install rules
   differ a bit

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