Add pickle.dumps tests
Converting to type api: PyObject_GC_New -> PyType_GenericAlloc
Converting to type api: put PyType_GenericAlloc into tp_alloc
Always initialize types, as I am going to use them in Python 2 as well
Replace module-level factory functions with proper constructors
Sphinx can now use method docstrings
Reinstate Curl* class documentation
Prohibit arguments to Curl* constructors, per the old API
tp_alloc is supposed to return zeroed memory, rely on this fact.

This commit adds assertions that the memory is in fact zeroed.
In the future the assertions should only be checked in debug mode.
util_curl_new is now unnecessary
Trashcan API is meant for DECREF calls, keep PyObject_GC_Del outside
Add semicolons for better looks
Move PyObject_GC_Del to tp_free
Changelog entry
Update docstring sources in makefile
Prohibit (de)serialization of Curl* objects.

This code seems to be necessary on Python 3.
On Python 2, serialization was already not working.