Run lint in Github Actions (#933)

Run lint in Github Actions to avoid PRs being merged that have
accidentally included linting violations.

Fix a file that has a linting violation.

Author: tevansuk

oauth2_provider/middleware.py

@@ -21,6 +21,7 @@ class OAuth2TokenMiddleware:
     It also adds "Authorization" to the "Vary" header, so that django's cache middleware or a
     reverse proxy can create proper cache keys.
     """
+
     def __init__(self, get_response):
         self.get_response = get_response
 

tox.ini

@@ -11,7 +11,7 @@ python =
     3.5: py35
     3.6: py36
     3.7: py37
-    3.8: py38, docs
+    3.8: py38, docs, flake8
     3.9: py39
 
 [pytest]