remix of pytest runner

[David Bremner]

I didn't love the idea of an evergrowing list of variables for
versions of python, since to add or delete a version we'd have to
update several places, even with the use of functions.  This version
uses one variable, which because configure is supposed to be fairly
portable is a space seperated list of names of python binaries to run.
It also replaces inlines functions into the bodies of loops, not for
efficiency but just to avoid some (perceived?) expansion subtleties.