Preliminary typing.py, anticipating provisional acceptance of PEP 484.

There area bunch of TODOs here, but the biggest (not mentioned in the
file) is that I'm going to take out __instancecheck__ and
__subclasscheck__.  However my personal schedule is such that I
probably won't have time for these before Larry tags beta 1.  But I
will try -- this commit is mostly to make sure that typing.py doesn't
completely miss the train.

PS. I'm tracking issues at https://github.com/ambv/typehinting/issues.

Doc/library/typing.rst

+:mod:`typing` --- Support for type hints
+========================================
+
+.. module:: typing
+   :synopsis: Support for type hints (see PEP 484).
+
+**Source code:** :source:`Lib/typing.py`
+
+--------------
+
+This module supports type hints as specified by :pep:`484`.  The most
+fundamental support consists of the type :class:`Any`, :class:`Union`,
+:class:`Tuple`, :class:`Callable`, :class:`TypeVar`, and
+:class:`Generic`.  For full specification please see :pep:`484`.  For
+a simplified introduction to type hints see :pep:`483`.

Misc/NEWS

@@ -58,6 +58,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #23973: PEP 484: Add the typing module.
+
 - Issue #20035: Replaced the ``tkinter._fix`` module used for setting up the
   Tcl/Tk environment on Windows with a private function in the ``_tkinter``
   module that makes no permanent changes to the environment.