Update to use ubuntu-latest

Author: gcatanese

.github/workflows/python-ci.yml

@@ -13,7 +13,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: ['3.6', '3.7', '3.8']