Rewrite generators based on feedback from Twitter

https://twitter.com/scaramagus/status/1085668552789708801

Author: humitos

readthedocs/core/views/serve.py

@@ -28,6 +28,7 @@
 from __future__ import (
     absolute_import, division, print_function, unicode_literals)
 
+import itertools
 import logging
 import mimetypes
 import os
@@ -287,11 +288,7 @@ def priorities_generator():
         iteration. After 0.1 is reached, it will keep returning 0.1.
         """
         priorities = [1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2]
-        for p in priorities:
-            yield p
-
-        while True:
-            yield 0.1
+        yield itertools.chain(priorities, itertools.repeat(0.1))
 
     def changefreqs_generator():
         """
@@ -301,11 +298,7 @@ def changefreqs_generator():
         will return always ``monthly``.
         """
         changefreqs = ['daily', 'weekly']
-        for c in changefreqs:
-            yield c
-
-        while True:
-            yield 'monthly'
+        yield itertools.chain(changefreqs, itertools.repeat('monthly'))
 
     sorted_versions = sort_version_aware(project.versions.filter(active=True))