CI: Downgrade runner OS to retain support for Python 3.6

Author: amotl

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-20.04, macos-latest, windows-latest]
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12-dev"]
         exclude: