flake8 fixes

Just a couple of flake8 fixes.

Fixes these issues:

    ./funtests/benchmarks/bench_worker.py:11:1: F401 'group' imported but unused
    ./funtests/benchmarks/bench_worker.py:74:5: E265 block comment should start with '# '
    ./celery/backends/__init__.py:13:1: F401 'urlparse' imported but unused