‘Upgrade’ from Python 2.7 to Python 3.7

Python is a non-issue.  But App Engine took the opportunity to change everything.

appengine/main.py

@@ -0,0 +1,43 @@
+"""
+Copyright 2020 Google LLC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+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.
+"""
+
+import cgi
+import storage
+from google.cloud import ndb
+
+
+# Datastore model.
+class Grid(ndb.Model):
+  data = ndb.StringProperty(required=True)
+
+
+# Route to requested handler.
+def app(environ, start_response):
+  if environ["PATH_INFO"] == "/":
+    return redirect(environ, start_response)
+  if environ["PATH_INFO"] == "/storage.py":
+    return storage.app(environ, start_response)
+  start_response("404 Not Found", [])
+  return [b"Page not found."]
+
+
+# Redirect for root directory.
+def redirect(environ, start_response):
+  headers = [
+    ("Location", "static/demos/index.html")
+  ]
+  start_response("301 Found", headers)
+  return []