title: ImportError: cannot import name 'bytes_type' in flask_oauthlib
body: <p>i try to build a apps with flask_oauthlib and deploy it to heroku, but i get some error</p>
  
  <pre><code>2019-02-10T03:14:55.112096+00:00 app[web.1]:   File "/app/flask_oauthlib/client.py", line 20, in &lt;module&gt;
  2019-02-10T03:14:55.112098+00:00 app[web.1]:     from .utils import to_bytes
  2019-02-10T03:14:55.112099+00:00 app[web.1]:   File "/app/flask_oauthlib/utils.py", line 5, in &lt;module&gt;
  2019-02-10T03:14:55.112102+00:00 app[web.1]:     from oauthlib.common import to_unicode, bytes_type
  2019-02-10T03:14:55.112106+00:00 app[web.1]: ImportError: cannot import name 'bytes_type'
  2019-02-10T03:14:55.112616+00:00 app[web.1]: [2019-02-10 03:14:55 +0000] [11] [INFO] Worker exiting (pid: 11)
  2019-02-10T03:14:55.112784+00:00 app[web.1]: [2019-02-10 03:14:55 +0000] [10] [INFO] Worker exiting (pid: 10)
  2019-02-10T03:14:55.267604+00:00 app[web.1]: [2019-02-10 03:14:55 +0000] [4] [INFO] Shutting down: Master
  2019-02-10T03:14:55.267763+00:00 app[web.1]: [2019-02-10 03:14:55 +0000] [4] [INFO] Reason: Worker failed to boot.
  2019-02-10T03:17:31.597353+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=askfess.herokuapp.com request_id=1e14f0da-5bbe-4849-9308-3ecf8a49306f fwd="140.213.38.39" dyno= connect= service= status=503 bytes= protocol=https
  </code></pre>
  
  <p>any ideas to fix it? the app error only when i try to deploy in heroku, but working fine in localhost</p>