Use Python 3 in script shebangs

Use Python 3 in script shebangs

(Skipping the two .pyx files with #!python - they need to be compiled
with Cython, not directly run with any version of plain Python)

Author: Rebecca N. Palmer <rebecca_palmer@zoho.com>
Forwarded: no

Gbp-Pq: Name python3_shebangs.patch