`typing_extensions` breaks Spack build

[garth-wells]

See https://github.com/FEniCS/dolfinx/actions/runs/5407842569/jobs/9826291336.

Is it essential?

[wence-]

I suppose not, it's a backport package for new type annotation features to older python versions. Is this happening because the spack build doesn't install dependencies in the same way? Or because we forgot to add it as a dependency in pyproject?

[garth-wells]

Dependencies need to be listed explicitly with Spack, see https://spack.readthedocs.io/en/2021.08.19/build_systems/pythonpackage.html#python-dependencies for the reasons.

[garth-wells]

typing_extensions is used to type hint very few 'private' methods, probably not justifying the introduction of a dependency. Easiest would be to remove. Docs can explain the type.