Fix compilation error under clang version 16

When compiled using clang v16, it issued the following error:
lru.c:629:17: error: incompatible function pointer types initializing 'PyCFunction' (aka 'struct _object *(*)(struct _object *, struct _object *)') with an expression of type 'PyCFunctionWithKeywords' (aka 'struct _object *(*)(struct _object *, struct _object *, struct _object *)') [-Wincompatible-function-pointer-types]
    {"popitem", (PyCFunctionWithKeywords)LRU_popitem, METH_VARARGS | METH_KEYWORDS,
The higher version clang will check exact type match for function pointer.

lru.c

@@ -626,7 +626,7 @@ static PyMethodDef LRU_methods[] = {
                     PyDoc_STR("L.setdefault(key, default=None) -> If L has key return its value, otherwise insert key with a value of default and return default")},
     {"pop", (PyCFunction)LRU_pop, METH_VARARGS,
                     PyDoc_STR("L.pop(key[, default]) -> If L has key return its value and remove it from L, otherwise return default. If default is not given and key is not in L, a KeyError is raised.")},
-    {"popitem", (PyCFunctionWithKeywords)LRU_popitem, METH_VARARGS | METH_KEYWORDS,
+    {"popitem", (PyCFunction)LRU_popitem, METH_VARARGS | METH_KEYWORDS,
                     PyDoc_STR("L.popitem([least_recent=True]) -> Returns and removes a (key, value) pair. The pair returned is the least-recently used if least_recent is true, or the most-recently used if false.")},
     {"set_size", (PyCFunction)LRU_set_size, METH_VARARGS,
                     PyDoc_STR("L.set_size() -> set size of LRU")},