Issue #20226: Major improvements to Argument Clinic.

* You may now specify an expression as the default value for a
  parameter!  Example: "sys.maxsize - 1".  This support is
  intentionally quite limited; you may only use values that
  can be represented as static C values.
* Removed "doc_default", simplified support for "c_default"
  and "py_default".  (I'm not sure we still even need
  "py_default", but I'm leaving it in for now in case a
  use presents itself.)
* Parameter lines support a trailing '\\' as a line
  continuation character, allowing you to break up long lines.
* The argument parsing code generated when supporting optional
  groups now uses PyTuple_GET_SIZE instead of PyTuple_GetSize,
  leading to a 850% speedup in parsing.  (Just kidding, this
  is an unmeasurable difference.)
* A bugfix for the recent regression where the generated
  prototype from pydoc for builtins would be littered with
  unreadable "=<object ...>"" default values for parameters
  that had no default value.
* Converted some asserts into proper failure messages.
* Many doc improvements and fixes.

Lib/test/test_inspect.py

@@ -1601,12 +1601,15 @@ class TestSignatureObject(unittest.TestCase):
         self.assertTrue(isinstance(signature, inspect.Signature))
         def p(name): return signature.parameters[name].default
         self.assertEqual(p('s'), 'avocado')
+        self.assertEqual(p('b'), b'bytes')
         self.assertEqual(p('d'), 3.14)
         self.assertEqual(p('i'), 35)
-        self.assertEqual(p('c'), sys.maxsize)
         self.assertEqual(p('n'), None)
         self.assertEqual(p('t'), True)
         self.assertEqual(p('f'), False)
+        self.assertEqual(p('local'), 3)
+        self.assertEqual(p('sys'), sys.maxsize)
+        self.assertEqual(p('exp'), sys.maxsize - 1)
 
     def test_signature_on_non_function(self):
         with self.assertRaisesRegex(TypeError, 'is not a callable object'):