Move setuptools to beginning of user-agent header. Fixes #598.

CHANGES.rst

@@ -2,6 +2,14 @@
 CHANGES
 =======
 
+v21.3.0
+-------
+
+* #598: Setuptools now lists itself first in the User-Agent
+  for web requests, better following the guidelines in
+  `RFC 7231
+  <https://tools.ietf.org/html/rfc7231#section-5.5.3>`_.
+
 v21.2.2
 -------
 

setuptools/package_index.py

@@ -202,8 +202,8 @@ def find_external_links(url, page):
             if match:
                 yield urllib.parse.urljoin(url, htmldecode(match.group(1)))
 
-user_agent = "Python-urllib/%s setuptools/%s" % (
-    sys.version[:3], require('setuptools')[0].version
+user_agent = "setuptools/%s Python-urllib/%s" % (
+    require('setuptools')[0].version, sys.version[:3],
 )
 
 class ContentChecker(object):