bpo-35059: Cleanup usage of Python macros (GH-10648)

Don't pass complex expressions but regular variables to Python
macros.

* _datetimemodule.c: split single large "if" into two "if"
  in date_new(), time_new() and datetime_new().
* _pickle.c, load_extension(): flatten complex "if" expression into
  more regular C code.
* _ssl.c: addbool() now uses a temporary bool_obj to only evaluate
  the value once.
* weakrefobject.c: replace "Py_INCREF(result = proxy);"
  with "result = proxy; Py_INCREF(result);"

Objects/weakrefobject.c

@@ -833,7 +833,8 @@ PyWeakref_NewProxy(PyObject *ob, PyObject *callback)
                        to avoid violating the invariants of the list
                        of weakrefs for ob. */
                     Py_DECREF(result);
-                    Py_INCREF(result = proxy);
+                    result = proxy;
+                    Py_INCREF(result);
                     goto skip_insert;
                 }
                 prev = ref;