Modify tox envs to simplify CI and add Python 3.8

.github/workflows/main.yml

@@ -17,19 +17,16 @@ jobs:
         config:
           - os: ubuntu
             python: 3.5
-            tox: py35
           - os: ubuntu
             python: 3.6
-            tox: py36
           - os: ubuntu
             python: 3.7
-            tox: py37
+          - os: ubuntu
+            python: 3.8
           - os: macos
             python: 3.7
-            tox: py37
           - os: windows
             python: 3.7
-            tox: py37
 
     steps:
       - uses: actions/checkout@v2
@@ -77,7 +74,7 @@ jobs:
           echo "=====  PANDOC  ====="
           pandoc --version | head -2
       - name: Run tests
-        run: tox -e ${{ matrix.config.tox }}
+        run: tox -e py${{ matrix.config.python }}
 
 
   lint: