Log Message:
Updated py-idna to 2.4.

2.4 (2017-03-01)
++++++++++++++++

- Restore IDNAError to be a subclass of UnicodeError, as some users of
  this library are only looking for the latter to catch invalid strings.

2.3 (2017-02-28)
++++++++++++++++

- Fix bugs relating to deriving IDNAError from UnicodeError.
- More memory footprint improvements (Thanks, Alex Gaynor)