Log Message:
Update py-idna to 2.1.

2.1 (2016-03-20)
++++++++++++++++

- Memory consumption optimizations. The library should consume significantly
  less memory through smarter data structures being used to represent
  relevant Unicode properties. Many thanks to Shivaram Lingamneni for this
  patch.
- Patches to make library work better with Python 2.6. The core library
  currently works however the unit testing does not. (Thanks, Robert
  Buchholz)
- Better affix all Unicode codepoint properties to a specific version.