Add count.reduce() (RustPython#3737)

Author: gnsxun

vm/src/stdlib/itertools.rs

@@ -193,7 +193,15 @@ mod decl {
     }
 
     #[pyimpl(with(IterNext, Constructor))]
-    impl PyItertoolsCount {}
+    impl PyItertoolsCount {
+        // TODO: Implement this
+        // if (lz->cnt == PY_SSIZE_T_MAX)
+        //      return Py_BuildValue("0(00)", Py_TYPE(lz), lz->long_cnt, lz->long_step);
+        #[pymethod(magic)]
+        fn reduce(zelf: PyRef<Self>) -> (PyTypeRef, (BigInt,)) {
+            (zelf.class().clone(), (zelf.cur.read().clone(),))
+        }
+    }
     impl IterNextIterable for PyItertoolsCount {}
     impl IterNext for PyItertoolsCount {
         fn next(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyIterReturn> {