bpo-36320: Switch typing.NamedTuple from OrderedDict to regular dict (GH-12396)

Also,  deprecate the *_field_types* attributes which duplicated the information in *\__annotations__*.


https://bugs.python.org/issue36320

Doc/library/typing.rst

@@ -838,10 +838,11 @@ The module defines the following classes, functions and decorators:
 
    Fields with a default value must come after any fields without a default.
 
-   The resulting class has two extra attributes: ``_field_types``,
-   giving a dict mapping field names to types, and ``_field_defaults``, a dict
-   mapping field names to default values.  (The field names are in the
-   ``_fields`` attribute, which is part of the namedtuple API.)
+   The resulting class has an extra attribute ``__annotations__`` giving a
+   dict that maps the field names to the field types.  (The field names are in
+   the ``_fields`` attribute and the default values are in the
+   ``_field_defaults`` attribute both of which are part of the namedtuple
+   API.)
 
    ``NamedTuple`` subclasses can also have docstrings and methods::
 
@@ -863,6 +864,15 @@ The module defines the following classes, functions and decorators:
    .. versionchanged:: 3.6.1
       Added support for default values, methods, and docstrings.
 
+   .. versionchanged:: 3.8
+      Deprecated the ``_field_types`` attribute in favor of the more
+      standard ``__annotations__`` attribute which has the same information.
+
+   .. versionchanged:: 3.8
+      The ``_field_types`` and ``__annotations__`` attributes are
+      now regular dictionaries instead of instances of ``OrderedDict``.
+
+
 .. function:: NewType(typ)
 
    A helper function to indicate a distinct types to a typechecker,