Won't install requirements.txt on Ubuntu 18.04 with python 3.6

[AntonOfTheWoods]

The version of uWSGI in requirements.txt doesn't seem to be able to compile anymore. I tried updating to 2.0.18, which enables it to install and show the addition form but I am getting errors when trying to submit.

Traceback (most recent call last):
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1997, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1985, in wsgi_app
    response = self.handle_exception(e)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1540, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/pylti/flask.py", line 192, in wrapper
    return function(*args, **kwargs)
  File "/home/a.melser/dev/mit_lti_flask_sample/mit_lti_flask_sample.py", line 97, in grade
    lti.post_grade(1 if correct else 0)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/pylti/common.py", line 606, in post_grade
    self.response_url, xml)
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/pylti/flask.py", line 98, in response_url
    url = self.session['lis_outcome_service_url']
  File "/home/a.melser/dev/venvs/flaskltiex/lib/python3.6/site-packages/werkzeug/local.py", line 368, in <lambda>
    __getitem__ = lambda x, i: x._get_current_object()[i]
KeyError: 'lis_outcome_service_url'

I also tried to deploy to heroku and that also fails - first because heroku-18 appears to only support Python 2.7.17 (for 2.7), requiring a runtime.txt update, and second for the same uWSGI error.

Changing these gets the heroku up and running but there are further errors. These may be the same - on heroku I just get There was an error, but the heroku cli won't install via snap and it is likely the same error there...

I was testing this on a local install of Open edX Ironwood (installed via tutor)