Create new port devel/py3-nose:

• This is a variant of devel/py-nose for Python 3.x. A separate port is needed until the ports tree gets VARIANTS support. The science/gramps port will soon be updated to a version which only supports python3, and it depends on py-nose with python3. To allow people to only use packages directly (and not ports), the package building infrastructure needs to build 2 packages: py-nose with python2 and py-nose with python3. Since the ports tree doesn't yet have VARIANTS support, this is not yet possible without making a separate py3-nose port.
• Ensure devel/py-nose does not clobber USES variable in slave devel/py3-nose
• Fix devel/py-nose target regression-test to use selftest.py [1]
  
  [1] https://github.com/nose-devs/nose/issues/400

PR: 207036
Reviewed by: mat (mentor)
Approved by: adamw (mentor), koobs (maintainer)
Differential Revision: https://reviews.freebsd.org/D6280