Revert "travis: use python3 for the autoconf builds"

This reverts commit 855af9a5a209f061355513b92f3ba4576f48d091.

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

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