Convert remaining python2 parts to python3

There are still a bunch of files containing python2 lines in their preamble:

./src/modules/testfds.py:#!/usr/bin/env python2
./src/relayselector.py:#!/usr/bin/env python2
./test/test_util.py:#!/usr/bin/env python2
./test/test_stats.py:#!/usr/bin/env python2
./test/run_tests.py:#!/usr/bin/env python2
./test/test_torsocks.py:#!/usr/bin/env python2
./test/test_relayselector.py:#!/usr/bin/env python2