gh-actions fails: 2.7 not available plus some black changes

[mauritsvanrees]

See scheduled run.

For 2.7 we probably need to specify a container. Or drop support. Copying a few lines from a buildout PR:

generate-scripts-py2:
    name: generate scripts - Python ${{ matrix.python-version }}
    strategy:
      matrix:
        package: [zest.releaser, pyspf]
        python-version: ["2.7"]
        os: ["ubuntu-20.04"]
    runs-on: ${{ matrix.os }}
    container:
      image: python:2.7.18-buster

I don't have time myself for the next few weeks, so if someone else wants to pick this up, I would appreciate it.

[mauritsvanrees]

We should drop support for Python 3.7 and earlier, Zope 4, and Plone 5.2. I will work on that.