bpo-37540: vectorcall: keyword names must be strings (GH-14682)

The fact that keyword names are strings is now part of the vectorcall and `METH_FASTCALL` protocols. The biggest concrete change is that `_PyStack_UnpackDict` now checks that and raises `TypeError` if not.

CC @markshannon @vstinner 


https://bugs.python.org/issue37540

Doc/c-api/object.rst

@@ -400,8 +400,8 @@ Object Protocol
    :c:func:`PyVectorcall_NARGS(nargsf) <PyVectorcall_NARGS>`.
 
    *kwnames* can be either NULL (no keyword arguments) or a tuple of keyword
-   names. In the latter case, the values of the keyword arguments are stored
-   in *args* after the positional arguments.
+   names, which must be strings. In the latter case, the values of the keyword
+   arguments are stored in *args* after the positional arguments.
    The number of keyword arguments does not influence *nargsf*.
 
    *kwnames* must contain only objects of type ``str`` (not a subclass),

Doc/c-api/structures.rst

@@ -204,6 +204,7 @@ also keyword arguments.  So there are a total of 6 calling conventions:
    Keyword arguments are passed the same way as in the vectorcall protocol:
    there is an additional fourth :c:type:`PyObject\*` parameter
    which is a tuple representing the names of the keyword arguments
+   (which are guaranteed to be strings)
    or possibly *NULL* if there are no keywords.  The values of the keyword
    arguments are stored in the *args* array, after the positional arguments.