Migrate to requests-cache

[niccokunzmann]

At the moment, the requests are cached using flask-caching.
However, there is a library that does exactly what we want: https://requests-cache.readthedocs.io

To close this issues:

• cache requests's responses of calendars using https://requests-cache.readthedocs.io
• adapt the tests to make sure the check that package
• while at it: evaluate the responses module and if it is useful for us.