Update etstool for use with Python 3.8

[mdickinson]

This PR updates etstool.py and the EDM-based test workflow to support Python 3.8 as well as Python 3.6.

• Updates etstool.py
• Updates the EDM-based PR-test workflow
• Updates the EDM-based test-with-ETS-source cron-job workflow

The etstool.py changes ended up being more extensive than I originally intended, since I ran into issues with using a non-EDM Python as a bootstrap environment on Windows. So there's now some additional logic to identify the EDM executable on Windows, overridable through a command-line argument or an environment variable. (This logic was copied from other similar workflows, like the one in Envisage.)

Checklist

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

[mdickinson]

(successful) ets-from-source run on this branch: https://github.com/enthought/apptools/actions/runs/2860939442