Please unblock package python-tz

[ Reason ]

python-tz contains a hard-coded list of timezones. This causes problems
on updates to tzdata. Last example: tzdata 2021a-1+deb11u9 added
America/Ciudad_Juarez but due to the hard-coded list, this timezone is
not selectable. Following code will fail with `UnknownTimeZoneError`:

```
#!/usr/bin/python3
import pytz
pytz.timezone("America/Ciudad_Juarez")
```

See also https://github.com/stub42/pytz/issues/91

[ Impact ]
Newly added timezones from tzdata will be availble to python3-tz.

[ Tests ]
I added autopkgtest cases to ensure that updates to tzdata do not
introduce regressions.

[ Risks ]

Several Python projects use python3-tz. The autopkgtest should reduce
the risk of regressions of dynamically determine the list of timezones.
python3-tz can fail in case they system has problems or tzdata is not
properly set-up. Python 3.9 ships the zoneinfo module which can be used
as replacement.

[ Checklist ]
  [x] all changes are documented in the d/changelog (except the
      formatting change by wrap-and-sort)
  [x] I reviewed all changes and I approve them
  [x] attach debdiff against the package in testing

unblock python-tz/2022.7.1-3

Unblocked.