bpo-8525: help() on a type now shows builtin subclasses (GH-5066)

For builtin types with builtin subclasses, help() on the type now shows up
to 4 of the subclasses. This partially replaces the exception hierarchy
information previously displayed in Python 2.7.

Lib/pydoc.py

@@ -1254,6 +1254,24 @@ location listed above.
                 push('    ' + makename(base))
             push('')
 
+        # List the built-in subclasses, if any:
+        subclasses = sorted(
+            (str(cls.__name__) for cls in object.__subclasses__()
+             if not cls.__name__.startswith("_") and cls.__module__ == "builtins"),
+            key=str.lower
+        )
+        no_of_subclasses = len(subclasses)
+        MAX_SUBCLASSES_TO_DISPLAY = 4
+        if subclasses:
+            push("Built-in subclasses:")
+            for subclassname in subclasses[:MAX_SUBCLASSES_TO_DISPLAY]:
+                push('    ' + subclassname)
+            if no_of_subclasses > MAX_SUBCLASSES_TO_DISPLAY:
+                push('    ... and ' +
+                     str(no_of_subclasses - MAX_SUBCLASSES_TO_DISPLAY) +
+                     ' other subclasses')
+            push('')
+
         # Cute little class to pump out a horizontal rule between sections.
         class HorizontalRule:
             def __init__(self):