Tighten lower bounds for dependencies

[rossbar]

The version windows for dependencies are extremely wide, extending well into versions that are no longer supported.

This generally shouldn't be an issue for those using workflows that create new environments, but can result in non-working environments for users who are getting old versions of packages; see e.g. #134.

Some things to improve the situation:

• Tighten lower bounds. This will largely be dictated by the minimum supported Python version.
• Add a procedure to the release process that involves testing against the oldest-supported versions of dependencies.

This should be done across the deepcell libraries.