use /usr/bin/env to reach python3

doc/doxygen/dev/starpu_check_documented.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 # StarPU --- Runtime system for heterogeneous multicore architectures.
 #
 # Copyright (C) 2013,2014,2016,2017                      CNRS

tools/starpu_mpi_comm_matrix.py.in

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 # StarPU --- Runtime system for heterogeneous multicore architectures.
 #