use self.payload[:self.payload_len] for getting python string

This is more Cythonic way, documented here
http://cython.readthedocs.io/en/latest/src/tutorial/strings.html#passing-byte-strings

It compiles to
    __Pyx_PyBytes_FromStringAndSize(
        __pyx_v_self->payload + 0,
        __pyx_v_self->payload_len - 0)

Which is the Cython macro wrapper which handles both Python 2 and
Python3.

Fixes #17

netfilterqueue.pyx

@@ -100,12 +100,7 @@ cdef class Packet:
     def get_payload(self):
         """Return payload as Python string."""
         cdef object py_string
-        if cpython.version.PY_MAJOR_VERSION >= 3:
-            py_string = PyBytes_FromStringAndSize(
-                self.payload, self.payload_len)
-        else:
-            py_string = PyString_FromStringAndSize(
-                self.payload, self.payload_len)
+        py_string = self.payload[:self.payload_len]
         return py_string
 
     cpdef Py_ssize_t get_payload_len(self):