Test another version

Author: Shylpx

.github/workflows/cartoframes-ci.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: ubuntu-20.04
+          - os: ubuntu-18.04
             python-version: 3.5
           - os: ubuntu-20.04
             python-version: 3.6