Ci fix

.github/workflows/python-ci.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       matrix:
         python-version: ["3.5", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
-        os: [ windows-latest, macos-latest, ubuntu-20.04 ]
+        os: [ windows-latest, macos-latest, ubuntu-latest ]
         exclude:  # Python < v3.8 does not support Apple Silicon ARM64.
           - python-version: "3.5"
             os: macos-latest
@@ -37,7 +37,19 @@ jobs:
     steps:
     - uses: actions/checkout@v4
 
+    - name: Run tests in Docker for legacy Python
+      if: matrix.os == 'ubuntu-latest' && contains(fromJSON('["3.5","3.6","3.7"]'), matrix.python-version)
+      run: |
+        docker run --rm -v ${{ github.workspace }}:/app -w /app python:${{ matrix.python-version }} bash -c "
+          pip install --upgrade pip &&
+          pip install pytest pytest-cov parameterized mock flake8 &&
+          pip install -r requirements.txt &&
+          flake8 configcatclient --count --show-source --statistics &&
+          pytest configcatclienttests
+        "
+
     - name: Set up Python ${{ matrix.python-version }}
+      if: ${{ !(matrix.os == 'ubuntu-latest' && contains(fromJSON('["3.5","3.6","3.7"]'), matrix.python-version)) }}
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
@@ -46,25 +58,29 @@ jobs:
         PIP_TRUSTED_HOST: "pypi.python.org pypi.org files.pythonhosted.org"
 
     - name: Install dependencies
+      if: ${{ !(matrix.os == 'ubuntu-latest' && contains(fromJSON('["3.5","3.6","3.7"]'), matrix.python-version)) }}
       run: |
         python -m pip install --upgrade pip 
         pip install pytest pytest-cov parameterized mock flake8
         pip install -r requirements.txt
 
     - name: Lint with flake8
+      if: ${{ !(matrix.os == 'ubuntu-latest' && contains(fromJSON('["3.5","3.6","3.7"]'), matrix.python-version)) }}
       run: |
         # Statical analysis
         flake8 configcatclient --count --show-source --statistics
 
     - name: Test
+      if: ${{ !(matrix.os == 'ubuntu-latest' && contains(fromJSON('["3.5","3.6","3.7"]'), matrix.python-version)) }}
       run: pytest configcatclienttests
 
 
   coverage:
     needs: [ test ]
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+
       - name: Set up Python
         uses: actions/setup-python@v5
         with: