Change shebangs with Tools/scripts/pathfix.py?

There is a script in `/usr/lib64/python3.6/Tools/scripts/pathfix.py` that changes shebangs. We may want to:

* recommend that in the shebang sections of this guide
* use it automagically in rpm builds to fix shebangs? (with similar logic to pybytecompile script)

I'm opening this here, because of the first above point. If we say we want to use this automatically, we can discuss this in some ML.

Example usage:

```spec
%install
...
%{python3_sitearch}/Tools/scripts/pathfix.py -i "%{__python3} -s" -p %{buildroot}%{_bindir}/*
```

Note: A small problem with this script is that it only converts files where the name matches `r'^[a-zA-Z0-9_]+\.py$'` when called on directory, but you can call it directly on files.