chore: fix python 3.6 build

Python 3.6 is not included in Ubuntu 22. Rather than dropping support
for CI to test on 3.6, run the tests on Ubuntu 20.

actions/setup-python#544

Author: cbarton

.github/workflows/ci.yml

@@ -1,15 +1,20 @@
 name: CI
 on: [push]
-concurrency: 
+concurrency:
   group: ${{ github.ref }}
   cancel-in-progress: true
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     name: Python ${{ matrix.python }} tests
     strategy:
       matrix:
-        python: [2.7, 3.6, 3.7, 3.8, 3.9]
+        python: [2.7, 3.7, 3.8, 3.9]
+        os: [ubuntu-latest]
+        include:
+          # Python 3.6 is not included in ubuntu latest (currently 22)
+          - python: 3.6
+            os: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - name: Setup python