Make `visualizer.py` no longer itself a CGI

visualizer.py

-#!/usr/bin/env python3
-
 import datetime
 from io import BytesIO
 import json
 import math
 from types import SimpleNamespace
 from urllib.request import urlopen
-from wsgiref.handlers import CGIHandler
 
 # Activate the virtual env to get access to non-standard packages.
 activator = 'venv/bin/activate_this.py'
@@ -380,4 +377,4 @@ def read_data(station, year):
 
 
 if __name__ == '__main__':
-    CGIHandler().run(app)
+    app.run()