python: restore more compatible behavior for $PYTHON.

python: restore more compatible behavior for $PYTHON.

For https://bugs.gnu.org/74434.

* lib/py-compile: if $PYTHON -V does not include the
string "python" (case-insensitive), consider the support
intentionally disabled and exit successfully, unless PYTHON is set
to false, in which case exit unsuccessfully. This is closer to
the old behavior. Mention this in the help message.
* t/py-compile-env.sh: add test for PYTHON=:.
* NEWS: mention this. (And, en passant, add some past bug#s and
clarify that only RCS/SCCS pattern rules were disabled, not all.)