more portable python shebangs (#5816)

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.

src/basic/generate-gperfs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 """Generate %-from-name.gperf from %-list.txt
 """