Introduce pyupgrade

Closes #1742 (closed)

Introducing pyupgrade.

Again, I've made one commit per rule.

arkindex/images/models.py

@@ -218,8 +218,8 @@ class Image(S3FileMixin, IndexableModel):
         if max_width is None and max_height is None:
             resize = "full"
         else:
-            resize = "{0},{1}".format(max_width or "", max_height or "")
-        return "{0}/full/{1}/0/default.jpg".format(self.url.rstrip("/"), resize)
+            resize = "{},{}".format(max_width or "", max_height or "")
+        return "{}/full/{}/0/default.jpg".format(self.url.rstrip("/"), resize)
 
     @property
     def s3_put_url(self):