trivial modification of all shebang.

`/usr/bin/python3` -> `/usr/bin/env python3`

It's always better to trust the environment defined path to bin/python, as
virtualenv become the way to go.

tests/test_corrupted_files.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import unittest
 import shutil

tests/test_libmat2.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import unittest
 import shutil

tests/test_policy.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import unittest
 import shutil