Feature or enhancement

Migrate _zoneinfo.c to argument clinic.

It is proposed by @vstinner and @pganssle
Link: #19909 (comment)

Pitch

This will improve several things:

1. help() on ZoneInfo methods. Here's how it looks now:

>>> help(zoneinfo.ZoneInfo.clear_cache)
Help on built-in function clear_cache:

clear_cache(...) method of builtins.type instance
    Clear the ZoneInfo cache.

1. Add __text_signature__ property to methods, it is very useful: for example, python/typeshed uses it to be sure that stubs and implementation match
2. Move all argument parsing to clinic, which is more convenient

Previous discussion

1. PEP 615: Add zoneinfo module #84683
2. bpo-40503: Add tests and implementation for ZoneInfo #19909
3. bpo-40503: Add tests and implementation for ZoneInfo #19909 (comment)

I will send a PR for this today :)