Issue #26778: Fixed "a/an/and" typos in code comment and documentation.

Doc/c-api/set.rst

@@ -128,7 +128,7 @@ or :class:`frozenset` or instances of their subtypes.
    of brand new frozensets before they are exposed to other code).  Return 0 on
    success or -1 on failure. Raise a :exc:`TypeError` if the *key* is
    unhashable. Raise a :exc:`MemoryError` if there is no room to grow.  Raise a
-   :exc:`SystemError` if *set* is an not an instance of :class:`set` or its
+   :exc:`SystemError` if *set* is not an instance of :class:`set` or its
    subtype.
 
 
@@ -142,7 +142,7 @@ subtypes but not for instances of :class:`frozenset` or its subtypes.
    error is encountered.  Does not raise :exc:`KeyError` for missing keys.  Raise a
    :exc:`TypeError` if the *key* is unhashable.  Unlike the Python :meth:`~set.discard`
    method, this function does not automatically convert unhashable sets into
-   temporary frozensets. Raise :exc:`PyExc_SystemError` if *set* is an not an
+   temporary frozensets. Raise :exc:`PyExc_SystemError` if *set* is not an
    instance of :class:`set` or its subtype.
 
 
@@ -150,7 +150,7 @@ subtypes but not for instances of :class:`frozenset` or its subtypes.
 
    Return a new reference to an arbitrary object in the *set*, and removes the
    object from the *set*.  Return *NULL* on failure.  Raise :exc:`KeyError` if the
-   set is empty. Raise a :exc:`SystemError` if *set* is an not an instance of
+   set is empty. Raise a :exc:`SystemError` if *set* is not an instance of
    :class:`set` or its subtype.