Dependabot updates don't propagate to secondary env files

[liamhuber]

Dependabot triggers updates in modules with versions specified in setup.py, and these get automatically propagated to .ci_support/environment.yml, and from there on to the .binder/environment.yml and docs/environment.yml just fine.

However, dependabot also nicely catches updates to optional dependencies in setup.py, but these are not getting propagated to, e.g., .ci_support/environment-notebooks.yml.

I haven't looked yet how clever our current merge script is; my hope is that we can straightforwardly set it up so that extras_require={"some_extras_key": [...]} can get automatically merged with .ci_support/environment-some_extras_key.yml. IMO that would be a pretty sensible interface.

I know dependabot interacts just fine with pyproject.toml, but I don't yet know the details of how so some modifications to these ideas may be needed for compatibility there.