- Use tox-uv to speed up installations in tox environments.
- While doing so, adopt the new practice of pinning the tox versions
  that's running for consistency.
- Use a PYTHON_VERSION environment variable in GitHub Actions to make it
  more consistent to update.

Now that tox-docker randomizes its container's ports for reliability,
the configuration needs to pick up the environment variables provided by
tox-docker. Safir does this with the special EnvAsyncPostgresDsn and
EnvRedisDsn connection URL types that look for tox-docker environment
variables.

The expression can be returned directly.

This isn't needed now that redis has its own type annotations.

This is an optimization from Gafaelfawr

This is the modern way of dumping to JSON-serializable data from
Pydantic 2.