attempting to make CI work for python 3.6

.github/workflows/ci.yaml

@@ -21,7 +21,20 @@ jobs:
       fail-fast: false
       matrix:
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
-        os: [ubuntu-latest]
+        os: ["ubuntu-latest", "ubuntu-20.04"]
+        exclude:
+          - os: ubuntu-20.04
+            python-version: "3.7"
+          - os: ubuntu-20.04
+            python-version: "3.8"
+          - os: ubuntu-20.04
+            python-version: "3.9"
+          - os: ubuntu-20.04
+            python-version: "3.10"
+          - os: ubuntu-20.04
+            python-version: "3.11"
+          - os: ubuntu-20.04
+            python-version: "3.12"
 
     steps:
     - uses: actions/checkout@v4
@@ -36,9 +49,9 @@ jobs:
 
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
-        if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
-        if [ -f qa/requirements.txt ]; then pip install -r qa/requirements.txt; fi
+        python -m pip install --no-cache-dir --upgrade pip
+        if [ -f requirements.txt ]; then pip install --no-cache-dir -r requirements.txt; fi
+        if [ -f qa/requirements.txt ]; then pip install --no-cache-dir -r qa/requirements.txt; fi
 
     - name: Lint with flake8
       run: |