Revert "configure: allow building with python3"

Revert "configure: allow building with python3"

This reverts commit ae7898dfdbe5c8dab7d11c71862353f1ae43feb0.

Turns out the python scripts are _not_ fully python 3 compatible.
As Ilia reported using get_xmlpool.py with LANG=C produces some weird
output - see the link for details.

Even though the issue was spotted with the autoconf build, it exposes a
genuine problem with the script (and lack of lang handling of the meson
build.)

https://lists.freedesktop.org/archives/mesa-dev/2018-August/203508.html