Update equinox requirement from ^0.12.1 to ^0.13.1

Updates the requirements on [equinox](https://github.com/patrick-kidger/equinox) to permit the latest version.
- [Release notes](https://github.com/patrick-kidger/equinox/releases)
- [Commits](patrick-kidger/equinox@v0.12.1...v0.13.1)

---
updated-dependencies:
- dependency-name: equinox
  dependency-version: 0.13.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pyproject.toml

@@ -25,7 +25,7 @@ typer = "^0.15.1"
 jaxtyping = "^0.3.2"
 pydantic = "^2.10.6"
 pyyaml = "^6.0.2"
-equinox = "^0.12.1"
+equinox = "^0.13.1"
 
 [tool.poetry.scripts]
 covvfit = "covvfit.cli:main"