Fix an `except` for Py3

It appears to have been introduced with the merge of PR #28 and I guess PR #26 was created before this and didn't handle it.

I think we're not seeing the failure in Travis CI only because the Travis CI jobs are failing first with the problem addressed in https://github.com/linsomniac/python-memcached/pull/34

Cc: @linsomniac, @cabrera