(py-emacs-features): Test for working make-temp-name, which is broken

(py-emacs-features): Test for working make-temp-name, which is broken
on NTEmacs 19.34.6.

(py-serial-number): New variable.

(py-execute-region): If make-temp-name is broken, simply append a
serial number to the string "python-" to get a temporary file name.
It's possible concurrent NTEmacs can step on each others toes, but it
makes no sense to further coddle a busted NTEmacs.