Add missing import

Author: ruddyscent

Chapter4/weighted_graph.py

@@ -13,7 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from typing import TypeVar, Generic, List, Tuple
+from typing import TypeVar, Generic, List, Tuple, Optional
 from graph import Graph
 from weighted_edge import WeightedEdge