Add ``is_callable``, ``deep_iterable``, and ``deep_mapping`` validators.

* ``is_callable``: validates that a value is callable
* ``deep_iterable``: Allows recursion down into an iterable,
  applying another validator to every member in the iterable
  as well as applying an optional validator to the iterable itself.
* ``deep_mapping``: Allows recursion down into the items in a mapping object,
  applying a key validator and a value validator to the key and value in every item.
  Also applies an optional validator to the mapping object itself.

Add ``is_callable``, ``deep_iterable``, and ``deep_mapping`` validators.

* ``is_callable``: validates that a value is callable
* ``deep_iterable``: Allows recursion down into an iterable,
  applying another validator to every member in the iterable
  as well as applying an optional validator to the iterable itself.
* ``deep_mapping``: Allows recursion down into the items in a mapping object,
  applying a key validator and a value validator to the key and value in every item.
  Also applies an optional validator to the mapping object itself.

You can find them in the ``attr.validators`` package.

Add a pointless line to newsfragment to trick towncrier
Add a pointless line to newsfragment to trick towncrier

Otherwise we get invalid RST.
