chore: use v1 timezones API

[toph-allen]

Intent

Use v1 timezones API.

Because this is a really simple switch, I took the opportunity to think about how to handle checking the server version before trying the old request.

Currently not ready. Right now the server version isn't cached in any way so it results in two requests every time, and is no better than just trying the new URL first.

There are a bunch of existing functions around checking the server version, but they grouped "version not available" with "old version", which is the opposite of what we need, so I did some light refactoring to treat "version not available" as NULL and preserve that. I don't know if I like this approach though; maybe better to just not touch those functions and write a version as a dynamic, cached property of the server object.

Fixes #300

Approach

Explain how you solved the problem and any tradeoffs.

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Checklist

• Does this change update NEWS.md (referencing the connected issue if necessary)?
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