test against Ubuntu 20.04 to continue allowing testing with Python 3.6

Author: jlwalke2

.github/workflows/build-test-deploy.yml

@@ -22,7 +22,7 @@ jobs:
     strategy:
       matrix:
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10']
-        os-version: [ubuntu-20.04, windows-latest, macos-latest]
+        os-version: ['ubuntu-20.04', 'windows-latest', 'macos-latest']
 # Pinned Ubuntu version to 20.04 since no Python 3.6 builds available on ubuntu-latest (22.04) as of 2022-12-7.
 #        os-version: [ubuntu-latest, windows-latest, macos-latest]