[3.3] bpo-30500: urllib: Simplify splithost by calling into urlparse. (#1849) (#2292)

[3.3] bpo-30500: urllib: Simplify splithost by calling into urlparse. (#1849) (#2292)

The current regex based splitting produces a wrong result. For example::

  http://abc#@def

Web browsers parse that URL as ``http://abc/#@def``, that is, the host
is ``abc``, the path is ``/``, and the fragment is ``#@def``.
(cherry picked from commit 90e01e50ef8a9e6c91f30d965563c378a4ad26de)
(cherry picked from commit cc54c1c0d2d05fe7404ba64c53df4b1352ed2262)