Build on ubuntu-20.04

Author: Olivier Chédru

.github/workflows/build.yml

@@ -6,7 +6,7 @@ on:
         branches: [ '*' ]
 jobs:
     build:
-        runs-on: ubuntu-latest
+        runs-on: ubuntu-20.04
         strategy:
             matrix:
                 python-version: [ '2.7', '3.5', '3.6', '3.7', '3.8', '3.9', '3.10', '3.11' ]