CI: run on Ubuntu 20.04 to check Python 3.7

Author: mgeier

.github/workflows/python-versions.yml

@@ -11,7 +11,7 @@ jobs:
           - "3.10"
           - "pypy-3.7"
           - "pypy-3.8"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Install PortAudio
         run: |