TST: Fix NOQA for flake8 checking in datetools.py
Recent changes to pyflakes (see <a href="https://github.com/pyflakes/pyflakes/blob/0532189b34086740d7fe553bd87c0a6cf682a93b/pyflakes/checker.py#L544-L546">here</a>) suddenly started raising errors in `datetools.py` due to the star imports at the top, which the `#noqa` was not preventing. The correct syntax is `# flake8: noqa`  <br><br><b>PLEASE MERGE ASAP</b>

For anyone out there with permissions to cancel tests on Travis, all of the builds that are right now queued before mine for this PR (<a href="https://travis-ci.org/pydata/pandas/builds/113302752">#18011</a>) <b>WILL FAIL</b> because of this style checking issue.  At the very least, if someone could cancel all of my builds (excluding this one, just search for <b>"gfyoung"</b>) as well as cancel any old builds of anyone else's PR's so that this one can land ASAP, that would be great.
 there is more, already have these in #12511 

maybe we should use a fixed version of pyflakes
 Perhaps.  I was surprised to see that star import checking popping up relatively late in the game given how many versions have come out already, though discouraging star imports in the <i>general case</i> is understandable.
