scripts/maint: Stop assuming that python3 is in /usr/bin

Obviously correct fix to rename_c_identifier.py.

Bugfix on 32237; not in any released version of tor.

scripts/maint/rename_c_identifier.py

-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # Copyright (c) 2001 Matej Pfajfar.
 # Copyright (c) 2001-2004, Roger Dingledine.