bpo-31709: Drop support for asynchronous __aiter__. (#3903)

Doc/reference/datamodel.rst

@@ -2520,9 +2520,8 @@ generators, coroutines do not directly support iteration.
 Asynchronous Iterators
 ----------------------
 
-An *asynchronous iterable* is able to call asynchronous code in its
-``__aiter__`` implementation, and an *asynchronous iterator* can call
-asynchronous code in its ``__anext__`` method.
+An *asynchronous iterator* can call asynchronous code in
+its ``__anext__`` method.
 
 Asynchronous iterators can be used in an :keyword:`async for` statement.
 
@@ -2552,48 +2551,14 @@ An example of an asynchronous iterable object::
 
 .. versionadded:: 3.5
 
-.. note::
+.. versionchanged:: 3.7
+   Prior to Python 3.7, ``__aiter__`` could return an *awaitable*
+   that would resolve to an
+   :term:`asynchronous iterator <asynchronous iterator>`.
 
-   .. versionchanged:: 3.5.2
-      Starting with CPython 3.5.2, ``__aiter__`` can directly return
-      :term:`asynchronous iterators <asynchronous iterator>`.  Returning
-      an :term:`awaitable` object will result in a
-      :exc:`PendingDeprecationWarning`.
-
-      The recommended way of writing backwards compatible code in
-      CPython 3.5.x is to continue returning awaitables from
-      ``__aiter__``.  If you want to avoid the PendingDeprecationWarning
-      and keep the code backwards compatible, the following decorator
-      can be used::
-
-          import functools
-          import sys
-
-          if sys.version_info < (3, 5, 2):
-              def aiter_compat(func):
-                  @functools.wraps(func)
-                  async def wrapper(self):
-                      return func(self)
-                  return wrapper
-          else:
-              def aiter_compat(func):
-                  return func
-
-      Example::
-
-          class AsyncIterator:
-
-              @aiter_compat
-              def __aiter__(self):
-                  return self
-
-              async def __anext__(self):
-                  ...
-
-      Starting with CPython 3.6, the :exc:`PendingDeprecationWarning`
-      will be replaced with the :exc:`DeprecationWarning`.
-      In CPython 3.7, returning an awaitable from ``__aiter__`` will
-      result in a :exc:`RuntimeError`.
+   Starting with Python 3.7, ``__aiter__`` must return an
+   asynchronous iterator object.  Returning anything else
+   will result in a :exc:`TypeError` error.
 
 
 .. _async-context-managers: