Temporarily pin to ubuntu-20.04 to workaround actions/setup-python#544

Author: hugovk

.github/workflows/main.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   tests:
     name: "Python ${{ matrix.python-version }}"
-    runs-on: "ubuntu-latest"
+    runs-on: "ubuntu-20.04"
 
     strategy:
       fail-fast: false