Make OWSLib optional on Python 3

It seems like OWSLib might have a ways to go before it gets Python 3 support, whereas Cartopy is nearly ready _now_. So this PR makes OWSLib fully optional for Python 3 by:
 * Deferring import errors from import time up to the point where classes depending on OWSLib are used.
 * Skip OWSLib tests on Python 3
Skip if ``WebMapTileService is None``? At least then we only need to fix the import when it does work...