Don't Require pytz

[speedplane]

In this pull request, we don't explicitly require pytz unless the ical events use timezones. pytz is a huge library, and many often try to avoid it if they do not need it. In this case, if you're just using the library to generate all-day events, it isn't strictly necessary.

[mister-roboto]

@speedplane thanks for creating this Pull Request and help improve Plone!

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass.

Whenever you feel that the pull request is ready to be tested, either start all jenkins jobs pull requests by yourself, or simply add a comment in this pull request stating:

@jenkins-plone-org please run jobs

With this simple comment all the jobs will be started automatically.

Happy hacking!

[speedplane]

@jenkins-plone-org please run jobs

[niccokunzmann]

@speedplane, we are in the process of setting up maintenance for this repository again, see #360. It will take some time but I hope, we will get further towards integrating your changes. Thanks a lot for the effort and patience already!
If you like to run the latest tests for this PR, you can rebase/merge master.

[niccokunzmann]

from icalendar.parser import tzid_from_dt

Other modules might need modification now, after that long time.
Also, adding a test would be good: run a new file that imports icalendar, parses a file and checks that pytz is absent.

I quite like this PR as it allows that we support pytz and tzinfo in the future without requiring the import of pytz.

[niccokunzmann]

I will close this for now as it did not receive any attention towards changes. It inspired other issues! It can be opened again.