Merge port-systemd-7419: Port twisted.python.systemd to Python 3.

Author: hawkowl
Reviewer: Alex
Fixes: #7419

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@42747 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Add type(addr) = bytes support to isIPAddress and isIPv6Address

Added tests for isIPAddress
Port twisted.web.client.Agent functionality to Python 3
Port twisted.web.client.downloadPage to Python 3