Disable _SystemdParser on Python 3.x

twisted.python.systemd is not ported to Python 3.x, so importing
twisted.internet.endpoints is failed.
Fix LineReceiver.dataReceived on data without a delimiter on Python 3

self._buffer.split() may return a single result if no delimiter exists
in the buffer, resulting in ValueError: too many values to unpack on
Python 3
Run 2to3 on python/reflect.py

Otherwise it is impossible to import it on Python 3
Add Python 3 specific checks to isIPAddress and isIPv6Address
Port twisted.web.client.Agent functionality to Python 3