change get_user to use a protocol requiring a session (#522)

* change get_user to use a protocol requiring a session
define a "_HasSession" protocol, and update contrib.auth.get_user to use it
get_user only requires a session field, and idiomatic django testing frequently calls get_user with a TestClient

* run black

* use union for get_user instead of a protocol

* create tests for get_user typechecking

* properly import test client

tests/typecheck/contrib/auth/test_misc.yml

@@ -0,0 +1,15 @@
+-   case: test_request_session
+    main: |
+      from django.contrib.auth import get_user
+      from django.contrib.sessions.middleware import SessionMiddleware
+      from django.http import HttpRequest
+
+      request = HttpRequest()
+      SessionMiddleware().process_request(request)
+      get_user(request)
+-   case: test_client_session
+    main: |
+        from django.contrib.auth import get_user
+        from django.test import Client
+
+        get_user(Client())