Message 110085 - Python tracker

@ Éric and Antoine: Thanks for the useful hints!

* PATH and PATHEXT are now evaluated when needed, not only on module init. The rationale is, that the lib user may change them, eg. to include a directory with additional commands.

* the only helper module-level variables left are _windows and _getwinpathext() 

* updated doctest

* updated versionadded in docs to 3.2