Bump isort, use --diff (#164)

* dev-requirements: bump isort: 4.3.4 => 4.3.21

* isort: use --diff

* known_first_party = mypy_django_plugin

* Run isort

.travis.yml

@@ -30,7 +30,7 @@ jobs:
 
     - name: Lint plugin code with isort
       python: 3.7
-      script: 'isort --check'
+      script: 'isort --check --diff'
 
 before_install: |
   sudo apt update