External dependency `pytz` has already been removed from the main code,
but this PR also removes it from the tests. Why have I changed the test
timezone from CET to Europe/London? The tests were written with the [W3
examples](http://www.w3.org/TR/NOTE-datetime) in mind, but not
accounting for the summertime offset. As such, they originally assumed
that datetime(year=1997, month=7, day=16, hour=19, minute=20,
tzinfo=ZoneInfo("CET")) would evaluate to 1997-07-16T19:20+01:00. But it
doesn't; in summertime (July), CET is actually +02:00 and an example of
a +01:00 timezone would be "Europe/London". With the shift to the built
in ZoneInfo module, the test therefore needs to be adapted.
Get rid of the coerce_overrides() function.
Add the ParseOverrides argparse.Action to parse overrides.
Treat all extra settings values strictly as json values.
Test overrides.
Edit docs and cli help.