CI: run Python 2.7 tests in a container.

Temporary work-around for:

    ##[error]The version '2.7' with architecture 'x64' was not found for Ubuntu 20.04.

.github/workflows/ci.yml

@@ -29,13 +29,25 @@ jobs:
           os: ubuntu-22.04
         - python: '3.12-dev'
           os: ubuntu-22.04
+    container: ${{ (startsWith(matrix.python, '2.') && 'python:2.7-buster') || null }}
     runs-on: ${{matrix.os}}
     steps:
     - uses: actions/checkout@v3
     - name: set up Python ${{matrix.python}}
       uses: actions/setup-python@v4
       with:
         python-version: ${{matrix.python}}
+      if: ${{ ! startsWith(matrix.python, '2.') }}
+    - name: set up APT
+      run: |
+        printf '\n\nPackage: *\nPin: release o=Ubuntu\nPin-Priority: 9999\n' | sudo tee -a /etc/apt/preferences
+        printf 'Apt::Install-Recommends "false";\n' | sudo tee -a /etc/apt/apt.conf
+        sudo apt-get update
+      if: ${{ startsWith(matrix.python, '2.') }}
+    - name: upgrade TLS stack
+      run: |
+        sudo apt-get install --only-upgrade ca-certificates libgnutls30
+      if: ${{ startsWith(matrix.python, '2.') }}
     - name: install setuptools (if distutils is missing)
       run: |
         if ! [ -d ${{env.pythonLocation}}/lib/python*/distutils/ ]; then