Dashboard fails to start using Python 3

[tomason]

Operating environment/Installation (Hass.io/Docker/pip/etc.):

Fedora 29 armv7 (Banana Pi)
Python 3.7

ESP (ESP32/ESP8266, Board/Sonoff):

Not applicable

Affected component:

ESPHome dashboard

Description of problem:
The dashboard fails to start in 1.13 running on Python 3. It used to work with 1.12. The issue is easy to fix, there is a missing method in Python 3 (see bellow).

Problem-relevant YAML-configuration entries:
Not applicable

Traceback (if applicable):

Traceback (most recent call last):
  File "/home/esphome/.local/bin/esphome", line 11, in <module>
    sys.exit(main())
  File "/home/esphome/.local/lib/python3.7/site-packages/esphome/__main__.py", line 483, in main
    return run_esphome(sys.argv)
  File "/home/esphome/.local/lib/python3.7/site-packages/esphome/__main__.py", line 459, in run_esphome
    return PRE_CONFIG_ACTIONS[args.command](args)
  File "/home/esphome/.local/lib/python3.7/site-packages/esphome/__main__.py", line 348, in command_dashboard
    from esphome.dashboard import dashboard
  File "/home/esphome/.local/lib/python3.7/site-packages/esphome/dashboard/dashboard.py", line 189, in <module>
    class EsphomeCommandWebSocket(tornado.websocket.WebSocketHandler):
  File "/home/esphome/.local/lib/python3.7/site-packages/esphome/dashboard/dashboard.py", line 172, in websocket_class
    for _, method in cls.__dict__.iteritems():
AttributeError: 'mappingproxy' object has no attribute 'iteritems'

Additional information and things you've tried:
Replace line 172 in file esphome/dashboard/dashboard.py to say for _, method in cls.__dict__.items():