Revert "configure: allow building with python3"

This reverts commit ae7898dfdb.

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

src/compiler/glsl/glcpp/tests/glcpp-test.sh

@@ -13,4 +13,4 @@ if [ -z "$srcdir" -o -z "$abs_builddir" ]; then
     abs_builddir=`pwd`/../../../
 fi
 
-$PYTHON $srcdir/glsl/glcpp/tests/glcpp_test.py $abs_builddir/glsl/glcpp/glcpp $srcdir/glsl/glcpp/tests --unix --windows --oldmac --bizarro
+$PYTHON2 $srcdir/glsl/glcpp/tests/glcpp_test.py $abs_builddir/glsl/glcpp/glcpp $srcdir/glsl/glcpp/tests --unix --windows --oldmac --bizarro

src/compiler/glsl/tests/optimization-test.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-$PYTHON $srcdir/glsl/tests/optimization_test.py --test-runner $abs_builddir/glsl/glsl_test
+$PYTHON2 $srcdir/glsl/tests/optimization_test.py --test-runner $abs_builddir/glsl/glsl_test

src/compiler/glsl/tests/warnings-test.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-$PYTHON $srcdir/glsl/tests/warnings_test.py --glsl-compiler $abs_builddir/glsl_compiler --test-directory $srcdir/glsl/tests/warnings/
+$PYTHON2 $srcdir/glsl/tests/warnings_test.py --glsl-compiler $abs_builddir/glsl_compiler --test-directory $srcdir/glsl/tests/warnings/