Pytz recursive version conflict

[ashkuren]

Hello,

The project fixes pytz version to 2018.9 (mentiones via icalendar), whereas icalendar does not mention any specific pytz version in dependencies. This gives a dependency loop issue that can appear when icalendar is installed before recurring-ical-events (current pytz version is 2019.3).

Moreover, pytz is not used in the project itself, but only in tests. I think, it's better to leave only icalendar and dateutil as a dependency and move pytz without specific version to test-requirements.txt.

[niccokunzmann]

Hi and thanks for reporting this. Feel free to submit a pull request which fixes it in your view.