Use standard python implementation of which

Routine "which" from shutil appears to be faster, most likely because it pulls fewer dependencies that its e3.os.fs equivalent (that provides extra functionality that we don't need).

Author: ptroja

testsuite/gnatprove/lib/python/test_support.py

@@ -8,7 +8,7 @@
 import re
 import sys
 from time import sleep
-from e3.os.fs import which
+from shutil import which
 from e3.os.process import Run, STDOUT
 from test_util import sort_key_for_errors