Update the tool to use the latest version of Python 3

When updating the due dates for a lot of quizzes/assignments and a due date is after a availability date an error message comes up. However, the error message referrers to an ID # which does not show by the quizzes/assignments so there is no clear way to determine which one has the issue. See attached for the error message.

We should set up a CI pipeline (probably Travis) to run linters and tests. This will help catch easily-avoidable regressions like #24.

Also look into coverage reporting via a tool like Coveralls

Describe the bug

We've received reports from an instructor that when they go to update the dates for quizzes, the due dates don't change.

I am awaiting a response from the instructor with more details

DDC does not currently work with Ungraded Surveys and Practice Quizzes.

This presumably is because the tool is only looking at gradable assignments.

Add support for Ungraded Surveys and Practice Quizzes

Add the ability to change the due dates for an individual, section, or group.

Hello After updating to the latest version to get the chrome 80 cookies fix, we started noticing a 500 error when updating assignments. Here's the trace:

Traceback (most recent call last):
File "/ddchanger/env/local/lib/python2.7/site-packages/flask/app.py", line 2446, in wsgi_app
  response = self.full_dispatch_request()
File "/ddchanger/env/local/lib/python2.7/site-packages/flask/app.py", line 1951, in full_dispatch_request
  rv = self.handle_user_exception(e)
File "/ddchanger/env/local/lib/python2.7/site-packages/flask/app.py", line 1820, in handle_user_exception
  reraise(exc_type, exc_value, tb)
File "/ddchanger/env/local/lib/python2.7/site-packages/flask/app.py", line 1949, in full_dispatch_request
  rv = self.dispatch_request()
File "/ddchanger/env/local/lib/python2.7/site-packages/flask/app.py", line 1935, in dispatch_request
  return self.view_functions[rule.endpoint](**req.view_args)
File "/ddchanger/env/src/pylti/pylti/flask.py", line 385, in wrapper
  return function(*args, **kwargs)
File "/ddchanger/lti.py", line 189, in update_assignments
  if not request.is_xhr:
File "/ddchanger/env/local/lib/python2.7/site-packages/werkzeug/local.py", line 347, in __getattr__
  return getattr(self._get_current_object(), name)
AttributeError: 'Request' object has no attribute 'is_xhr'

To make it work I had to comment this line in lti.py that checks if the request is an ajax request.

While troubleshooting, I found that this was removed from the werkzeug package in v1.0.0.

Need to set our session cookies to specify SameSite=None; Secure

https://blog.chromium.org/2019/10/developers-get-ready-for-new.html

Safari won't save session cookies between pages on an iframe LTI that is hosted on a domain that is different from the parent window.

ie: duedatechanger.heroku.com loaded from canvas.myuniversity.edu won't work, BUT,

duedatechanger.myuniversity.edu loaded from canvas.myuniversity.edu WILL.