Support scripts properly

[henryiii]

See https://packaging.python.org/en/latest/specifications/binary-distribution-format/#recommended-installer-features & mesonbuild/meson-python#220. We should process scripts in /scripts to replace a nice shebang with the required one.

This is what distutils uses (and misses pypy):

shebang_pattern = re.compile('^#!.*python[0-9.]*([ \t].*)?$')

Also seems to not suppert pythonw. Though I'm not sure you'd indicate that in a source file that you wanted to run on Linux too.