[4.2.x] Added GitHub Action workflow to test all Python versions listed in the project config file.

Backport of 470f4c2436 from main.

.github/workflows/python_matrix.yml

@@ -0,0 +1,52 @@
+name: Python Matrix from config file
+
+on:
+  pull_request:
+    types: [labeled, synchronize, opened, reopened]
+    paths-ignore:
+      - 'docs/**'
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+   contents: read
+
+jobs:
+  define-matrix:
+    if: contains(github.event.pull_request.labels.*.name, 'python-matrix')
+    runs-on: ubuntu-latest
+    outputs:
+      python_versions_output: ${{ steps.set-matrix.outputs.python_versions }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - id: set-matrix
+        run: |
+          python_versions=$(sed -n "s/^.*Programming Language :: Python :: \([[:digit:]]\+\.[[:digit:]]\+\).*$/'\1', /p" setup.cfg | tr -d '\n' | sed 's/, $//g')
+          echo "Supported Python versions: $python_versions"
+          echo "python_versions=[$python_versions]" >> "$GITHUB_OUTPUT"
+  python:
+    runs-on: ubuntu-latest
+    needs: define-matrix
+    strategy:
+      matrix:
+        python-version: ${{ fromJson(needs.define-matrix.outputs.python_versions_output) }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          cache: 'pip'
+          cache-dependency-path: 'tests/requirements/py3.txt'
+      - name: Install libmemcached-dev for pylibmc
+        run: sudo apt-get install libmemcached-dev
+      - name: Install and upgrade packaging tools
+        run: python -m pip install --upgrade pip setuptools wheel
+      - run: python -m pip install -r tests/requirements/py3.txt -e .
+      - name: Run tests
+        run: python tests/runtests.py -v2