Bugfix: URLPath.is_leaf could sometimes return a non-boolean.
Major documentation overhaul.

Thanks to vmalloc for working on the initial preparation of Sphinx docs
and doctests. These have been extended to include a few more edge cases,
and full text descriptions. Now, every method and property on URLObject
is documented. Yay.
Bumped to v2.3.3.
Fixed print_function import issue.
Bumped to v2.3.4.
More doc work; incorporated the tour into the Sphinx docs.
More doc work.