When porting the change to Python 3.4, I found this older change:

     if _hostprog is None:
-        _hostprog = re.compile('^//([^/?]*)(.*)$')
+        _hostprog = re.compile('//([^/?]*)(.*)', re.DOTALL)
 
     match = _hostprog.match(url)
     if match:
-        host_port = match.group(1)
-        path = match.group(2)
-        if path and not path.startswith('/'):
+        host_port, path = match.groups()
+        if path and path[0] != '/':
             path = '/' + path
         return host_port, path

made by:

commit 44eceb6e2aca4e6d12ce64fa0f90279ffff19c25
Author: Serhiy Storchaka <storchaka@gmail.com>
Date:   Tue Mar 3 20:21:35 2015 +0200

    Issue #23563: Optimized utility functions in urllib.parse.

With this change, newlines are now accepted in URLs.

@Serhiy: Was it a deliberate change?