[Twisted-Python] Drop support for Python 3.4?

[Tobias Oberstein]

> Python 3.5 is still the default python3 version in Debian stable, so
> updating beyond that might make it more complex for Debian users to
> install Twisted.

yeah, that, and: as of today, pypy3 is at python 3.5

so _not_ supporting 3.5 would definitely be a problem for us (crossbar.io)

dropping 3.4: no problem with that, same with 3.3

dropping 2.7: np .. even though there are users of autobahn (a 
websocket/wamp library) still on py2.7, and we fully support that right 
now and have no plans (yet) to retire it

cheers,
/Tobias