Expose a zoneinfo package

[pganssle]

Right now, this is namespaced as backports.zoneinfo, but it would make the upgrade path much easier if we also installed a zoneinfo package in site-packages.

The way I see it is that backports.zoneinfo would be a running backport of the latest features for a while (at least until we decide that the module is more or less stable), and the zoneinfo module would be there so that you can write code that seamlessly upgrades to using the standard library module, rather than using something ungainly like:

try:
    import zoneinfo
except ImportError:
    from backports import zoneinfo

The biggest downsides I immediately see are:

1. Providing two packages is icky.
2. People may not realize that zoneinfo and backports.zoneinfo may point to two different things.
3. It may be mildly annoying to do the wheel and sdist builds if sometimes the package provides zoneinfo and backports.zoneinfo, and other times it provides just backports.zoneinfo.

Another option would be to create a second PyPI package, either zoneinfo-name or zoneinfo, explicitly mark it as python_requires >=3.6,< 3.8, and have it expose the backports.zoneinfo module as zoneinfo. I'm somewhat inclined to do it that way rather than put both in the same package, as it seems neater (though people may be confused about which one to install).