CI: Downgrade ubuntu image for py36 tests

Author: elacuesta

.github/workflows/tests.yml

@@ -11,22 +11,28 @@ on:
 jobs:
   tests-ubuntu:
     name: "Test: py${{ matrix.python-version }}, Ubuntu"
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
       matrix:
         include:
         - python-version: '3.6'
+          os: ubuntu-20.04
           tox-env: min
         - python-version: '3.6'
+          os: ubuntu-20.04
           tox-env: py
         - python-version: '3.7'
+          os: ubuntu-latest
           tox-env: py
         - python-version: '3.8'
+          os: ubuntu-latest
           tox-env: py
         - python-version: '3.9'
+          os: ubuntu-latest
           tox-env: py
         - python-version: '3.10'
+          os: ubuntu-latest
           tox-env: py
 
     steps: