Require apptools 5.3 or later, Python 3.8 or later

[mdickinson]

This PR updates the apptools dependency to apptools >= 5.3, and deals with various pieces of fallout from that change:

Detailed changes:

• require apptools >= 5.3, which contains some fixes that we need for some upcoming bugfixes on this repository (see #583)
• require apptools[preferences] rather than just plain apptools, since version 5.3.0 of apptools no longer declares configobj as an explicit dependency, and Envisage preferences depend on configobj
• similarly, require configobj explicitly in etstool.py, since EDS doesn't support optional requirements
• require Python >= 3.8, since apptools now also requires Python >= 3.8, and apptools >= 5.3.0 is not available for Python 3.7

[mdickinson]

It looks as though the tests are failing on EDM due to an incorrect version for importlib_resources. I've opened an upstream issue.

[mdickinson]

importlib_resources has been fixed upstream. Re-running the tests.

[mdickinson]

@flongford This PR should get us the green tick back on PR #580.

[mdickinson]

The PR title and description were misleading about the actual goal of this PR; I've updated them.

[mdickinson]

There's a required check for Python 3.7 that will need to be removed before merging.

[mdickinson]

There's a required check for Python 3.7 that will need to be removed before merging.

Done.