gh-119521: Remove _IncompleteInputError from the docs (GH-120993)

Lib/test/exception_hierarchy.txt

@@ -45,7 +45,6 @@ BaseException
       ├── StopAsyncIteration
       ├── StopIteration
       ├── SyntaxError
-      │    └── _IncompleteInputError
       │    └── IndentationError
       │         └── TabError
       ├── SystemError

Lib/test/test_baseexception.py

@@ -78,6 +78,9 @@ class ExceptionClassTests(unittest.TestCase):
                 last_depth = depth
         finally:
             inheritance_tree.close()
+
+        # Underscore-prefixed (private) exceptions don't need to be documented
+        exc_set = set(e for e in exc_set if not e.startswith('_'))
         self.assertEqual(len(exc_set), 0, "%s not accounted for" % exc_set)
 
     interface_tests = ("length", "args", "str", "repr")