Do test with Ubuntu 18.04

Sphinx-4.0 will drop support for Ubuntu 16.04.  So CI Platform should be
also updated to Ubuntu 18.04.

.github/workflows/main.yml

@@ -4,12 +4,11 @@ on: [push, pull_request]
 
 jobs:
   ubuntu:
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: false
       matrix:
-        name: [py36, py37, py38, py39]
-        os: [ubuntu-16.04]
+        name: [py36, py37, py38, py39, py310-dev]
         include:
         - name: py36
           python: 3.6
@@ -27,7 +26,6 @@ jobs:
         - name: py310-dev
           python: 3.10-dev
           docutils: du16
-          os: ubuntu-latest  # required
     env:
       PYTEST_ADDOPTS: ${{ matrix.coverage }}