Add User-Agent header when resolving via urllib (pex-tool#663)

At least some Python Index servers return a simple format of packages page, redirecting to / to /+simple/.

So there is a different result between `pex` run using `urllib` and using `requests` - in one case it errors.

pex/http.py

@@ -57,6 +57,7 @@ class Context(AbstractClass):
   """
 
   DEFAULT_ENCODING = 'iso-8859-1'
+  USER_AGENT = 'pex/%s' % PEX_VERSION
 
   class Error(Exception):
     """Error base class for Contexts to wrap application-specific exceptions."""
@@ -135,7 +136,8 @@ class UrllibContext(Context):
   """Default Python standard library Context."""
 
   def open(self, link):
-    return urllib_request.urlopen(link.url)
+    request = urllib_request.Request(link.url, headers={'User-Agent': self.USER_AGENT})
+    return urllib_request.urlopen(request)
 
   def content(self, link):
     if link.local:
@@ -146,7 +148,7 @@ def content(self, link):
       return fp.read().decode(encoding, 'replace')
 
   def resolve(self, link):
-    request = urllib_request.Request(link.url)
+    request = urllib_request.Request(link.url, headers={'User-Agent': self.USER_AGENT})
     request.get_method = lambda: 'HEAD'
     with contextlib.closing(urllib_request.urlopen(request)) as response:
       return link.wrap(response.url)
@@ -209,7 +211,6 @@ def close(self):
 
 class RequestsContext(Context):
   """A requests-based Context."""
-  USER_AGENT = 'pex/%s' % PEX_VERSION
 
   @staticmethod
   def _create_session(max_retries):