Fix GitHub Actions on Plone 5.2 Python 2.7.

[mauritsvanrees]

Fixes https://github.com/plone/buildout.coredev/issues/878

I have duplicated the existing yaml file, as this seemed easiest.

• One for Python 2.7, running explicitly on container image python:2.7.18-buster, which should fix the problem that Python 2.7 is not available anymore. No Windows or Mac.
• One for Python 3.8, still running on ubuntu-20.04, windows-latest, and macos-latest.
• No longer test unsupported Python 3.7.

[mauritsvanrees]

So: this fails on Python 3.8 Mac OS, and does not start any Python 2.7 jobs at all. Not very promising.

[mauritsvanrees]

GitHub actions are green again. We are now running tests on:

• Python 2.7 on ubuntu-20.04 (with container image python:2.7.18-buster)
• Python 3.8 on:
  • ubuntu-20.04
  • windows-2022
  • macos-11