- Add missing python-six BuildRequires

- Add missing python-six Requires

- Implement single-spec version.

- Update to 4.3.2
* using :meth:`_traitname_default()` without the ``@default``
decorator is no longer deprecated.
* Passing ``config=True`` in traitlets constructors is no longer
deprecated.
- Update to 4.3.1
* Compatibility fix for Python 3.6a1
* Fix bug in Application.classes getting extra entries when
multiple Applications are instantiated in the same process.
- Update to 4.3.0
* Improve the generated config file output.
* Allow TRAITLETS_APPLICATION_RAISE_CONFIG_FILE_ERROR env to
override :attr:`Application.raise_config_file_errors`,
so that config file errors can result in exiting immediately.
* Avoid using root logger. If no application logger is registered,
the ``'traitlets'`` logger will be used instead of the root
logger.
* Change/Validation arguments are now Bunch objects, allowing
attribute-access, in addition to dictionary access.
* Reduce number of common deprecation messages in certain cases.
* Ensure command-line options always have higher priority than
config files.
* Add bounds on numeric traits.
* Improves various error messages.