Remove misleading first line in test_rebind.py

test_rebind.py actually does not require Python 3.

Bugfix on 5a11670fcaad0a58de48425ba80510effbe35628; bug not in any
Tor release.

src/test/test_rebind.py

@@ -1,5 +1,3 @@
-#!/usr/bin/python3
-
 from __future__ import print_function
 
 import sys