FIX do not override Pickler's dispatch_table (#203)

If a Pickler instance already has methods in its dispatch table, do not
override them. This bug showed up in
cloudpipe/cloudpickle#253, where the
Cloudpickler instances derives from the C Pickler, and most custom
saving methods are inside dispatch_table, not dispatch.

loky/backend/reduction.py

@@ -188,7 +188,11 @@ def __init__(self, writer, reducers=None, protocol=HIGHEST_PROTOCOL):
             if sys.version_info < (3,):
                 self.dispatch = self._dispatch.copy()
             else:
-                self.dispatch_table = self._dispatch_table.copy()
+                if getattr(self, "dispatch_table", None) is not None:
+                    self.dispatch_table.update(self._dispatch_table.copy())
+                else:
+                    self.dispatch_table = self._dispatch_table.copy()
+
             for type, reduce_func in reducers.items():
                 self.register(type, reduce_func)