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

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

[flongford]

Generally LGTM - just a couple of comments on the scope of this issue.

I presume that these updated dependencies will be propagated through to our "publish on pip" GH actions workflow without further changes required. Do we have any "publish on EDS" equivalent workflows (automated or manual) that need to be updated post-merge?

[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.