Force Ubuntu 20 for 3.6 test runs (3.6 is EOL in Ubuntu latest)

Author: nbarrientos

.github/workflows/build.yml

@@ -4,11 +4,15 @@ on: [push]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7, 3.8, 3.9]
+        os: [ubuntu-latest]
+        include:
+          - python-version: 3.6
+            os: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2