bpo-41724: Explain when the conversion is not possible with detect_types enabled (GH-23855) (GH-23862)

* Explain when the conversion is not possible with detect_types enabled
(cherry picked from commit 09a36cdfb7)

Co-authored-by: sblondon <sblondon@users.noreply.github.com>

Co-authored-by: sblondon <sblondon@users.noreply.github.com>

Doc/library/sqlite3.rst

@@ -197,7 +197,9 @@ Module functions and constants
 
    *detect_types* defaults to 0 (i. e. off, no type detection), you can set it to
    any combination of :const:`PARSE_DECLTYPES` and :const:`PARSE_COLNAMES` to turn
-   type detection on.
+   type detection on. Due to SQLite behaviour, types can't be detected for generated
+   fields (for example ``max(data)``), even when *detect_types* parameter is set. In
+   such case, the returned type is :class:`str`.
 
    By default, *check_same_thread* is :const:`True` and only the creating thread may
    use the connection. If set :const:`False`, the returned connection may be shared

Doc/library/types.rst

@@ -260,6 +260,9 @@ Standard names are defined for the following types:
 
    .. versionadded:: 3.9
 
+   .. versionchanged:: 3.9.2
+      This type can now be subclassed.
+
 
 .. class:: TracebackType(tb_next, tb_frame, tb_lasti, tb_lineno)
 

Doc/whatsnew/3.9.rst

@@ -1497,3 +1497,22 @@ functions and options conditionally available based on the operating system
 version in use at runtime ("weaklinking").
 
 (Contributed by Ronald Oussoren and Lawrence D'Anna in :issue:`41100`.)
+
+Notable changes in Python 3.9.2
+===============================
+
+collections.abc
+---------------
+
+:class:`collections.abc.Callable` generic now flattens type parameters, similar
+to what :data:`typing.Callable` currently does.  This means that
+``collections.abc.Callable[[int, str], str]`` will have ``__args__`` of
+``(int, str, str)``; previously this was ``([int, str], str)``.  To allow this
+change, :class:`types.GenericAlias` can now be subclassed, and a subclass will
+be returned when subscripting the :class:`collections.abc.Callable` type.
+Code which accesses the arguments via :func:`typing.get_args` or ``__args__``
+need to account for this change.  A :exc:`DeprecationWarning` may be emitted for
+invalid forms of parameterizing :class:`collections.abc.Callable` which may have
+passed silently in Python 3.9.1.  This :exc:`DeprecationWarning` will
+become a :exc:`TypeError` in Python 3.10.
+(Contributed by Ken Jin in :issue:`42195`.)