Make CI run on Ubuntu 20.04 (#115)

This is temporary until we stop supporting Django 3.2. This will happen long before Ubuntu 20.04 goes out of fashion.

Author: kytta

.github/workflows/test.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
     name: build (Python ${{ matrix.python-version }}, Django ${{ matrix.django-version }})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: