Remove unnecessary `object` base class in docs (#10366).

Also add a note about inheritance from `object` being default.
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index d7a37c3..a52824f 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -1334,7 +1334,7 @@
 
    from multiprocessing.managers import BaseManager
 
-   class MathsClass(object):
+   class MathsClass:
        def add(self, x, y):
            return x + y
        def mul(self, x, y):