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.

Modules/posixmodule.c

@@ -2401,7 +2401,7 @@ class dir_fd_converter(CConverter):
 
 /*[clinic input]
 
-os.stat -> object(doc_default='stat_result')
+os.stat
 
     path : path_t(allow_fd=True)
         Path to be examined; can be string, bytes, or open-file-descriptor int.
@@ -2523,7 +2523,7 @@ posix_lstat(PyObject *self, PyObject *args, PyObject *kwargs)
     #define OS_ACCESS_DIR_FD_CONVERTER dir_fd_unavailable
 #endif
 /*[clinic input]
-os.access -> object(doc_default='True if granted, False otherwise')
+os.access
 
     path: path_t(allow_fd=True)
         Path to be tested; can be string, bytes, or open-file-descriptor int.