Add CPython 3.8 to GitHub Actions

Author: altendky

.github/workflows/ci.yml

@@ -37,6 +37,9 @@ jobs:
           - name: CPython 3.7
             tox: py37
             action: 3.7
+          - name: CPython 3.8
+            tox: py38
+            action: 3.8
         reactor:
           - name: default
             tox: default