Apply black formatter to code

[Travis] add a linting stage (via `black`)
[Travis] update as per code review
Remove "Lint" as separate stage; have `black` check the output rather
than run the re-formmater

.travis.yml

@@ -53,6 +53,13 @@ jobs:
     - os: windows
 
   include:
+    - stage: Lint & Tests
+      name: Lint, via Black
+      python: 3.8
+      # black is automatically installed by peotry
+      script:
+        - black --check . --verbose --diff
+
     # Python 3.6 Tests
     - name: Python 3.6 on Linux
       python: 3.6