ctypes: do not re-lock the destination in PyCData_set()

PyCData_set() already requires its caller to hold the critical section
on the destination object and asserts it, but finished by calling
KeepRef(), which takes that lock again.  Every store of a Python object
into a Structure or Union field, an Array item or a Pointer item went
through it.  Call KeepRef_lock_held() instead.

Measured on the free-threaded build: Structure int field assignment
38 -> 24 ns, c_char_p field assignment 69 -> 54 ns.

(cherry picked from commit ead0976)

Author: johng

Modules/_ctypes/_ctypes.c

@@ -3563,7 +3563,7 @@ PyCData_set(ctypes_state *st,
     /* KeepRef steals a refcount from it's last argument */
     /* If KeepRef fails, we are stumped.  The dst memory block has already
        been changed */
-    return KeepRef(mem, index, result);
+    return KeepRef_lock_held(mem, index, result);
 }