tests/check-gendoc

py3: replace hard coded `python` with $PYTHON in check-gendoc

#!/bin/sh
HGENCODING=UTF-8
export HGENCODING

echo ".. -*- coding: utf-8 -*-" > gendoc.txt
echo "" >> gendoc.txt
LANGUAGE=$1 "$PYTHON" "$TESTDIR/../doc/gendoc.py" >> gendoc.txt 2> /dev/null || exit

echo "checking for parse errors"
"$PYTHON" "$TESTDIR/../doc/docchecker" gendoc.txt
"$PYTHON" "$TESTDIR/../doc/runrst" html gendoc.txt /dev/null