Cleanup unused imports caught by pyflakes.
Make "import pyczmq" faster (backwards incompatible change!).

The basic "import pyczmq" goes from ~1.2 sec to 0.03 sec. Of course
that's not very realistic, since no one wants just the empty package.

A more realistic example: "from pyczmq import zmq" is now 0.36 sec
(from 1.2 sec). And "from pyczmq import zmq, zctx, zsocket" is now
0.53 sec.

This is also more in keeping with generally accepted practice in the
Python community (at least as I read things).

The catch is that this can break existing imports. Specifically, any
code like this:

  import pyczmq

  ctx = pyczmq.zctx.new()
  socket = pyczmq.zsocket.new(...)

WILL BREAK. The single import will have to be replaced by

  import pyczmq.zctx
  import pyczmq.zsocket

(or "from pyczmq import zctx, zsocket" if you don't mind changing your
code too).