Make 3.6 Python builds run on Ubuntu 20.04

Author: kytta

.github/workflows/test.yml

@@ -5,12 +5,16 @@ on: [push, pull_request]
 jobs:
   build:
     name: build (Python ${{ matrix.python-version }}, Django ${{ matrix.django-version }})
-    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', '3.11']
         django-version: ['3.2', '4.0', '4.1', 'main']
+        os: ['ubuntu-latest']
+        include:
+          - os: 'ubuntu-20.04'
+          - python-version: '3.6'
+    runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v2