[3.12] gh-91051: fix segfault when using all 8 type watchers (GH-107853) (#107876)

* gh-91051: fix segfault when using all 8 type watchers (GH-107853)
(cherry picked from commit 66e4edd)

Co-authored-by: Carl Meyer <carl@oddbird.net>

Author: miss-islington

Doc/c-api/typeobj.rst

@@ -147,7 +147,7 @@ Quick Reference
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
    | :c:member:`~PyTypeObject.tp_vectorcall`        | :c:type:`vectorcallfunc`          |                   |   |   |   |   |
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
-   | [:c:member:`~PyTypeObject.tp_watched`]         | char                              |                   |   |   |   |   |
+   | [:c:member:`~PyTypeObject.tp_watched`]         | unsigned char                     |                   |   |   |   |   |
    +------------------------------------------------+-----------------------------------+-------------------+---+---+---+---+
 
 .. [#slots]
@@ -2141,7 +2141,7 @@ and :c:data:`PyType_Type` effectively act as defaults.)
    .. versionadded:: 3.9 (the field exists since 3.8 but it's only used since 3.9)
 
 
-.. c:member:: char PyTypeObject.tp_watched
+.. c:member:: unsigned char PyTypeObject.tp_watched
 
    Internal. Do not use.