Use datetime functions from python standard lib

[ekuler]

The date handling functionality in Python std lib has been improved in Python 3 so we can take advantage of it and remove dependency on dateutil package.

This PR simplifies time zone handling for the various entry and group time values and creates time zone aware values everywhere.

Note that the parse function from dateutils is replaced by .fromisoformat() function from datetime class. However, this would raise the required Python version to 3.7. I believe this should be fine since 3.7 is quite old version and all major currently supported LTS Linux distributions are already on Python >=3.7.

If there is need to stay at Python 3.6, this can be rewritten in slightly more verbose way while still depending only on python standard library.

Using the strptime function now to stay compatible with Python 3.6 and to handle the time zone string .

[ekuler]

Replaced the fromisoformat function with strptime. Turns out, prior to Python 3.11, fromisoformat won't parse string produced with isoformat when timezone is specified. With this change, it should be compatible all the way to 3.6 again.

[ekuler]

The CI now just fails on lxml build for 3.11. This will be resolved when #351 gets merged.

[Evidlo]

Thanks! It's always good to get rid of dependencies.