Hard `setuptools_scm[toml]` dependency version required?

`pyproject.toml` has a hard version dependency for `setuptools_scm[toml]`, see https://github.com/pyamg/pyamg/blob/80bb0a7912ebd89d24e68112ae9e9fc9c4837a78/pyproject.toml#L5. 

Is this hard version dependency necessary? Hard dependencies can be a pain for package managers, e.g. Spack.