Add obspec as runtime dependency?

[kylebarron]

It's cool that it can be a dev-only dependency, and it's not actually used at runtime, but if it's not installed then the user won't get type hints in their editor.

This will be really nice if we get default extras: https://peps.python.org/pep-0771/

[kylebarron]

I've added tests that validate that obstore classes are assignable to obspec protocols, so we don't need to make obspec a dependency here.