CI: Test against development versions of core dependencies

[rossbar]

An extension of #636. IMO it's necessary to add additional testing against the development branches of some core dependencies (e.g. numpy, scipy, scikit-image, etc.). While we already test against the stable versions and catch DeprecationWarnings for incremental maintenance, there can be situations where DeprecationWarnings are not raised. See e.g. the discussion in #675.

The (IMO) best way to catch situations like this and fix them before they affect released code is to set up scheduled jobs to test against development versions of dependencies.