py-compile: fix optimized compiling for Python 3.5+

py-compile: fix optimized compiling for Python 3.5+

Fixes automake bug https://bugs.gnu.org/38043.

Split the optimized compilation logic into a new section.  This avoids
trying to support multiple versions of major versions in a single script
as it gets harder to verify new changes don't break old versions as time
goes on.

Now for Python 3.5+, compile with -O0 (which is "higher" than -O).

* NEWS: Mention fix.
* THANKS: Add Michal Górny.
* lib/py-compile: Add new section for compiling Python 3.5+.