Update test.yml

Updating ubuntu os version for python 3.6

Author: Akshaykdubey

.github/workflows/test.yml

@@ -11,14 +11,20 @@ name: Build & Test
   workflow_dispatch:
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
         python-version:
           - 3.6
           - 3.7
           - 3.8
           - 3.9
+        include:
+          - os: "ubuntu-latest"
+          # There is no build for Python 3.6 in ubuntu>20.04.
+          # This unclude can be removed when we drop support for Python 3.6.
+          - os: "ubuntu-20.04"
+            python-version: "3.6"
     steps:
       - uses: actions/checkout@v2
         with: