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

Also add a note about inheritance from `object` being default.
diff --git a/Doc/library/inspect.rst b/Doc/library/inspect.rst
index f81e157..4845bca 100644
--- a/Doc/library/inspect.rst
+++ b/Doc/library/inspect.rst
@@ -604,7 +604,7 @@
 code execution::
 
    # example code for resolving the builtin descriptor types
-   class _foo(object):
+   class _foo:
        __slots__ = ['foo']
 
    slot_descriptor = type(_foo.foo)