upgrade Flask-Themes2, fix SyntaxError

[neon-ninja]

This PR fixes the following issues:

Traceback (most recent call last):
  File "run.py", line 4, in <module>
    from cert_viewer import configure_app
  File "/home/ubuntu/cert-viewer/cert_viewer/__init__.py", line 6, in <module>
    from flask_themes2 import (Themes)
  File "/usr/local/lib/python3.8/dist-packages/flask_themes2/__init__.py", line 36, in <module>
    from werkzeug import cached_property
ImportError: cannot import name 'cached_property' from 'werkzeug' (/usr/local/lib/python3.8/dist-packages/werkzeug/__init__.py)

See jarus/flask-testing#143 for more info on this problem. flask-themes2 fixed this in sysr-q/flask-themes2@ea872ec, so upgrading to a version of flask-themes2 newer than Feb 13 2020 should fix this. https://pypi.org/project/Flask-Themes2/#history would suggest 0.1.5 or 1.0.0 would work.

Traceback (most recent call last):
  File "run.py", line 16, in <module>
    main()
  File "run.py", line 10, in main
    configure_app(conf)
  File "/home/ubuntu/cert-viewer/cert_viewer/__init__.py", line 56, in configure_app
    views.add_rules(app, configuration)
  File "/home/ubuntu/cert-viewer/cert_viewer/views/__init__.py", line 66, in add_rules
    from cert_viewer.views.request_view import RequestView
  File "/home/ubuntu/cert-viewer/cert_viewer/views/request_view.py", line 10, in <module>
    from cert_viewer.notifier import Notifier
  File "/home/ubuntu/cert-viewer/cert_viewer/notifier.py", line 59
    async=False)
    ^
SyntaxError: invalid syntax

See https://bitbucket.org/mailchimp/mandrill-api-python/commits/95106841ba96027825313912ea8f212db0059389 for more info on this problem. Fixes #75 and #77