Here is an updated patch based on Antoine's latest patch.

Summary of changes:
 * Updated docstrings of Pickler and Unpickler in the pickle module.
 * Fixed pickle._Pickler to consider fix_imports only for protocol < 3
 * Made module name remapping in _pickle more robust:
    - added PyUnicode_Check on global_name and module_name;
    - used PyDict_GetItemWithError instead of PyDict_GetItem
 * Changed Py_BuildValue("(OO)", ...) to its faster equivalent
   PyTuple_Pack(2, ...).

I don't really the idea of remapping names generated by Pickler, since
it breaks the identity guarantee in save_global(). However, I agree this
is an example where practicality beats purity. So, I do not oppose to
the change.