gh-127750: Restore inspect and pydoc support of singledispatchmethod (GH-130309)

The code is still flawed, because it does not recognize class and static
methods, and the first argument is not removed from the signature of
bound methods, but at least it does not worse than in 3.13 and older.

Lib/inspect.py

@@ -447,7 +447,8 @@ def isroutine(object):
             or isfunction(object)
             or ismethod(object)
             or ismethoddescriptor(object)
-            or ismethodwrapper(object))
+            or ismethodwrapper(object)
+            or isinstance(object, functools._singledispatchmethod_get))
 
 def isabstract(object):
     """Return true if the object is an abstract base class (ABC)."""