core: fix for working with python 3.3

The unfixed parse_querystring() raises the following exception
in Python 3.3:

  TypeError: Type str doesn't support the buffer API

Also removed iteritems() as it is deprecated and no longer
supported in Python 3.x

---

The same (or just very similar?) problem is mentioned in https://github.com/gabrielfalcao/HTTPretty/pull/27/files

The original exception:

            try:
                self.sock.sendall(data)
            except TypeError:
                if isinstance(data, collections.Iterable):
                    for d in data:
    >                   self.sock.sendall(d)

    /usr/lib64/python3.3/http/client.py:875:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    self = <httpretty.core.fakesock.socket object at 0x7fe1197e6a90>, data = 71, args = (), kw = {}

        def sendall(self, data, *args, **kw):
            self._sent_data.append(data)

            try:
    >           requestline, _ = data.split(b'\r\n', 1)
    E           AttributeError: 'int' object has no attribute 'split'

    ../../env/test/lib/python3.3/site-packages/httpretty-0.7.0-py3.3.egg/httpretty/core.py:346: AttributeError