pythonPackages.trio: fix build (#46342)

Fixes the build for `python3Packages.trio' for the next ZHF iteration.
Please refer to the Hydra build for further reference: https://hydra.nixos.org/build/80617356

`python3Packages.sniffio` is needed for the build, otherwise the build
aborts with an error like this:

```
  Could not find a version that satisfies the requirement sniffio (from trio==0.6.0) (from versions: )
No matching distribution found for sniffio (from trio==0.6.0)
```

See #45960