Make TraitsUI, Pyface and configobj optional dependencies

[mdickinson]

Apptools contains several disparate packages with different needs. While it's somewhat reasonable to regard Traits as a core dependency, not all subpackages need a GUI, and only the preferences package makes use of configobj.

This PR:

• makes all three of TraitsUI, Pyface and configobj optional dependencies
• adds a "gui" entry for extras_require in setup.py
• adds a test workflow that runs the test suite with only the core dependencies (i.e., Traits), to make sure that other tests are skipped appropriately rather than failing

Closes #70

Makes #86 obsolete.

Checklist

• [x] Add a news fragment if this PR is news-worthy for end users. (see docs/releases/README.rst)

[mdickinson]

I presume that these updated dependencies will be propagated through to our "publish on pip" GH actions workflow without further changes required.

Yes - the setup.py should take care of that.

Do we have any "publish on EDS" equivalent workflows (automated or manual) that need to be updated post-merge?

We don't: the "publish on EDS" workflow is entirely manual (I go and fill out a build request). So that build request will include a note that the requirements have changed.