Add support for Python 3.14

[hugovk]

Fixes #123.

The problem is in httpx's dependency httpcore. This was fixed in httpcore 1.0.8:

encode/httpcore#1005

However, httpx 0.24.1 used to set an upper bound on its httpcore dependency:

httpcore>=0.15.0,<0.18.0",

https://github.com/encode/httpx/blob/fcf1bc73dbe13bc61d18a6e998237a5021d3341c/pyproject.toml#L32

The latest httpx 0.28.1 allows us to used the fixed httpcore:

"httpcore==1.*",

https://github.com/encode/httpx/blob/26d48e0634e6ee9cdc0533996db289ce4b430177/pyproject.toml#L32

So let's use a newer httpx.

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Shall I also update the Poetry lockfile?