5.0.0 release of Traits

This major release accumulates more than an year's worth of improvements,
changes and bug fixes to the code base.

A few highlights of this release are :

• Removal of 2to3 fixers and the use of six to provide Python 2/3 compatibility
• Removal of deprecated traits.protocols submodule and related utils.
• New HasRequiredTraits class
• Better IPython tab completion for HasTraits subclasses

See the changelog for the complete list of changes included in this release.