Use IDNA2008 to encode hosts.

requests/models.py

@@ -21,6 +21,7 @@ from .structures import CaseInsensitiveDict
 
 from .auth import HTTPBasicAuth
 from .cookies import cookiejar_from_dict, get_cookie_header, _copy_cookie_jar
+from .packages import idna
 from .packages.urllib3.fields import RequestField
 from .packages.urllib3.filepost import encode_multipart_formdata
 from .packages.urllib3.util import parse_url
@@ -363,8 +364,8 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
 
         # Only want to apply IDNA to the hostname
         try:
-            host = host.encode('idna').decode('utf-8')
-        except UnicodeError:
+            host = idna.encode(host).decode('utf-8')
+        except (UnicodeError, idna.IDNAError):
             raise InvalidURL('URL has an invalid label.')
 
         # Carefully reconstruct the network location