New HTTP status codes (draft only).

See IETF document at
https://tools.ietf.org/html/draft-nottingham-http-new-status-02.

requests/status_codes.py

@@ -56,6 +56,9 @@ _codes = {
     424: ('failed_depdendency', 'depdendency'),
     425: ('unordered_collection', 'unordered'),
     426: ('upgrade_required', 'upgrade'),
+    428: ('precondition_required', 'precondition'),
+    429: ('too_many_requests', 'too_many'),
+    431: ('header_fields_too_large', 'fields_too_large'),
     444: ('no_response', 'none'),
     449: ('retry_with', 'retry'),
     450: ('blocked_by_windows_parental_controls', 'parental_controls'),