CI: Use ubuntu-20.04 to re-add support for Python 3.6

Author: amotl

.github/workflows/test.yml

@@ -26,10 +26,10 @@ concurrency:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest]
+        os: ["ubuntu-20.04"]
         python-version: [
           '3.6', '3.7', '3.8', '3.9', '3.10', '3.11',
           'pypy-3.6', 'pypy-3.7', 'pypy-3.8', 'pypy-3.9',