Clarify production readiness

[roskakori]

Currently the project is marked as "4 - Beta" on PyPI but the REAME says:

This package was originally the reference implementation for PEP 615

Python 3.9 has zoneinfo included and been around for a while now.

So is this a maintained backport intended for production use or an unmaintained "prove of concept" from back in the day? I'm fine with any and appreciate your work, but I think PyPI and README should make this perfectly clear.

The main reason for asking is that Django 4.0 deprecated pytz (still works but won't in Django 5.0) and suggests to switch to zoneinfo. However the current widely used Ubuntu 20 LTS still ships with Python 3.8 as default, and Ubuntu 22 LTS is not due before April 2022 (and even then it's advisable to wait a couple of months for it to ripen).

So this backport would be a nice option to use Django 4.0 on Ubuntu 20 LTS without pytz as opposed to more drastic measures like upgrading a 20 LTS to Python 3.9 or 3.10 or migrating to zoneinfo only after migrating to 22 LTS.