Remove unnecessary import.  (Fixes Python 3.3 reference problem.)

The import sys line is unnecessary because we have already imported it at the
global scope earlier in the file. This prevents a very strange reference leak
which I do not quite understand. The leak manifests itself in Python 3.3 as a
failing test (test_attrs_data.py: TestTypes.test_complex).

The bug was tracked down by turning on `gc.set_debug(gc.DEBUG_LEAK)` and
noticing an entry in `gc.garbage`::

   ImportError("No module named 'h5py.sys'",)

Certainly this should not have caused an import error, so it is potentially a
bug in Cython. More investigation will be required.

Closes #275.