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 +29,15 @@ 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: install setuptools (if distutils is missing)
       run: |
         if ! [ -d ${{env.pythonLocation}}/lib/python*/distutils/ ]; then
@@ -52,6 +54,7 @@ jobs:
         wget https://lcamtuf.coredump.cx/afl/releases/afl-latest.tgz -O deps/afl.tar.gz
         tar -xvzf deps/afl.tar.gz -C deps/
         make -C deps/afl-*/ install PREFIX=~/.local
+      container: null
     - name: install Cython
       run:
         python -m pip install --verbose ${{matrix.cython || 'cython'}}