contrib: port doxygen script to Python3

contrib/ChangeLog:

	* filter_gcc_for_doxygen: Use python3 and not python2.
	* filter_params.py: Likewise.

contrib/filter_gcc_for_doxygen

@@ -8,5 +8,5 @@
 # process is put on stdout.
 
 dir=`dirname $0`
-python $dir/filter_params.py $1
+env python3 $dir/filter_params.py $1
 exit 0

contrib/filter_params.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 """
 Filters out some of the #defines used throughout the GCC sources:
 - GTY(()) marks declarations for gengtype.c