Flask conversion

[frafra]

Support Python 3 and CKAN 2.9.
Work based on #42.

There are a couple of things I am not totally comfortable with, regarding the headers/https options and the missing database initialization command.

I tried to log in as an existing user via OAuth 2 on CKAN ~2.9.5 with Python 3, and it worked.

[frafra]

It seems that the original PR is active again. Great! Closing mine.

[simao-silva]

I am using CKAN 2.9.5 with Python 3. When using this PR I get the error `AttributeError: 'Request' object has no attribute 'GET''. Any ideas in what could be the issue?
Here is the full traceback:

 Traceback (most recent call last):
   File "/srv/app/src/ckanext-oauth2/ckanext/oauth2/views.py", line 59, in callback
     user_name = oauth2helper.identify(token)
   File "/srv/app/src/ckanext-oauth2/ckanext/oauth2/oauth2.py", line 132, in identify
     access_token = bytes(token['access_token'])
 TypeError: string argument without an encoding
 
 During handling of the above exception, another exception occurred:
 
 Traceback (most recent call last):
   File "/usr/lib/python3.8/site-packages/flask/app.py", line 1949, in full_dispatch_request
     rv = self.dispatch_request()
   File "/usr/lib/python3.8/site-packages/flask/app.py", line 1935, in dispatch_request
     return self.view_functions[rule.endpoint](**req.view_args)
   File "/srv/app/src/ckanext-oauth2/ckanext/oauth2/views.py", line 71, in callback
     error_description = toolkit.request.GET.get('error_description')
   File "/usr/lib/python3.8/site-packages/werkzeug/local.py", line 347, in __getattr__
     return getattr(self._get_current_object(), name)
   File "/usr/lib/python3.8/site-packages/werkzeug/local.py", line 347, in __getattr__
     return getattr(self._get_current_object(), name)
 AttributeError: 'Request' object has no attribute 'GET'