Fix heroku port use

app.py

@@ -1,9 +1,12 @@
 from flask import Flask
+import os
 import urllib.request
 from icalendar.cal import Calendar, Component
 
 app = Flask(__name__)
 
+port = int(os.environ.get('PORT', 33507))
+
 
 @app.route("/")
 def hello():
@@ -44,4 +47,4 @@ def hello():
     return other.to_ical().replace(b'\r\n', b'\n').strip()
 
 if __name__ == "__main__":
-    app.run()
+    app.run(port=port)