CI: upgrade to Python 3.9 (and 3.6 as minimum supported version)

Author: mgeier

.github/workflows/main.yml

@@ -8,16 +8,16 @@ jobs:
       matrix:
         include:
         - os: ubuntu-latest
-          python-version: 3.8
+          python-version: 3.9
           jack: jackd2
         - os: ubuntu-latest
-          python-version: 3.5
+          python-version: 3.6
           jack: jackd2
         - os: ubuntu-latest
-          python-version: 3.8
+          python-version: 3.9
           jack: jackd1
         - os: macos-latest
-          python-version: 3.8
+          python-version: 3.9
     runs-on: ${{ matrix.os }}
     steps:
     - name: Install JACK on Ubuntu
@@ -56,7 +56,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Double-check Python version
       run: |
         python --version