Move `dataclasses` to standard imports

visualizer.py

+from dataclasses import make_dataclass
 import datetime
 from io import BytesIO
 import json
@@ -5,7 +6,6 @@ from types import SimpleNamespace
 from urllib.request import urlopen
 
 import asccol
-from dataclasses import make_dataclass
 from flask import abort, Flask, g, jsonify, render_template, request, Response
 import matplotlib
 import matplotlib.pyplot as plt