Adjust for and require icalendar>=6.0.0

[iamleot]

This PR makes khal compatible with icalendar 6.0.0 and newer (and start to require it!).

In particular:

• For timezone functionalities icalendar can use both pytz and zoneinfo, preferring zoneinfo. We rely on pytz for several attributes, stick to it. This fixes Support icalendar v6 #1361 and should obsolete Replace tz.localize with date.replace #1363.
• icalendar.windows_to_olson is no longer available, we need to refer to it as icalendar.timezone.windows_to_olson

With this PR all the tests passes again with icalendar>=6.0.0.

[iamleot]

Leaving it as a draft because I need to RTFS of icalendar because maybe we need bump the version requirement and/or make it agnostic both to old and new icalendar.

use_pytz() appeared in icalendar-6.0.0 I have bumped the version requirement as well. Also icalendar.windows_to_olson is now exposed via icalendar.timezone.windows_to_olson from 6.0.0. If you think it is better to make the patch agnostic I think we can check if that function is available or not and I can try to adjust that too.

[iamleot]

The «docs/readthedocs.org:khal — Read the Docs build failed!» failures are probably unrelated to that. That's also tracked via #1356.

[d7415]

icalendar 6.0.0 was only released at the end of September, so may not have propagated as far as we'd like.

Other than that, I like it.

[iamleot]

icalendar 6.0.0 was only released at the end of September, so may not have propagated as far as we'd like.

Mmm, that's a good point.

Maybe we can try to do some dance as follows...

For use_pytz() we can add:

try:
    icalendar.use_pytz()
except AttributeError:
    # assumes icalendar<6.0.0, only pytz is used
    pass

While for icalendar.windows_to_olson vs icalendar.timezone.windows_to_olson we can do:

try:
    from icalendar.timezone import windows_to_olson
except ImportError:
    from icalendar import windows_to_olson

...and then replace the icalendar.windows_to_olson to just windows_to_olson.

(Beware, both only written as comment, not really tested!)

But, in particular the latter... IMHO is not very nice!

However, if you think it's worth to be more version agnostic I'm happy to adjust the patch as described in this comment!

Other than that, I like it.

Thank you!

[d7415]

Those could certainly work. Remembering to take them about later would be a pain.

There's nothing strictly wrong with "New khal needs icalendar >=6. If you don't have it stick with the last version of khal"

I think it's best to see if Christian or Hugo have an opinion on this (deliberately not tagging them for now)