The script wasn't apparently used since the switch to meson, because
it required the sys subdirectory to be present in the same subdirectory
where the output script is located.
Let's use f-strings to make the whole thing more readable. Add some
extra checks.
This is useful on systems like NixOS, where python3 is not in
/usr/bin/python3 as well as for people using alternative ways to
install python such as virtualenv/pyenv.
Instead of keeping all sys/ nodes in a tarball, use a script
"sys-script.py" to create all the sys/ entries.
This adds a script to create that initial "sys-script.py" script, using
an existing sys/ directory, created from the sys.tar.xz contents.
The "sys-script.py" can then be edited or recreated later, when any sys/
files are added or modified; the change will be only a patch to the
"sys-script.py" script in git, instead of forcing git to store a new
binary tarball.
