Fix flake8 error.

... and upgrade flake8 so that it works with the other flake8 plugins.

requires/testing.txt

@@ -1,6 +1,6 @@
 coverage==4.5.4
 codecov==2.0.15
-flake8==3.7.8
+flake8==3.7.9
 flake8-comprehensions==2.2.0
 flake8-deprecated==1.3
 flake8-html==0.4.0

sprockets/mixins/http/__init__.py

@@ -360,9 +360,8 @@ class HTTPClientMixin:
 
         # Fail hard if someone is doing something wrong
         if 'raise_error' in kwargs:
-            raise RuntimeError(
-                self.__class__.__name__ + '.http_fetch called with ' +
-                'raise_error')
+            raise RuntimeError(self.__class__.__name__ + '.http_fetch '
+                               'called with raise_error')
 
         for attempt in range(0, max_http_attempts):
             LOGGER.debug('%s %s (Attempt %i of %i) %r',