Python3 compatibility fixes for flask-session

[apakulov-stripe]

This PR addressing the following issues:

• flask.ext.session -> flask_session fixes ImportError: No module named 'flask.ext' as the old format flask.ext.* is deprecated, see: Deprecate flask.ext and create migration tool to flask_* pallets/flask#1135 (comment)
• Version bump from 0.2.1 to 0.2.3 address number of issues with Python 3, see changeset: https://github.com/fengsp/flask-session/blob/master/CHANGES#L31:L37. Here is also a stacktrace I've encountered when I was running confidant with flask-session 0.2.1 with Python 3.7 and Redis as a session storage.

 [2020-02-07 23:06:02.669077] ERROR:confidant.app:Exception on /v1/services [GET]
 [2020-02-07 23:06:02.669105] Traceback (most recent call last):
 [2020-02-07 23:06:02.669214]   File "/vendor3/lib/python3.7/site-packages/flask/app.py", line 2445, in wsgi_app
 [2020-02-07 23:06:02.669260]     ctx.push()
 [2020-02-07 23:06:02.669311]   File “/vendor3/lib/python3.7/site-packages/flask/ctx.py", line 390, in push
 [2020-02-07 23:06:02.669351]     self.session = session_interface.open_session(self.app, self.request)
 [2020-02-07 23:06:02.669608]   File "/vendor3/lib/python3.7/site-packages/flask_session/sessions.py", line 118, in open_session
 [2020-02-07 23:06:02.669662]     val = self.redis.get(self.key_prefix + sid)
 [2020-02-07 23:06:02.669714] TypeError: can only concatenate str (not "bytes") to str
 [2020-02-07 23:06:02.669755] ERROR:confidant.app:Request finalizing failed with an error while handling an error

[ryan-lane]

Thanks for the PR! Taking a look.