Create new port graphics/py3-pillow:

• This is a variant of graphics/py-pillow 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-pillow with python3. To allow people to only use packages directly (and not ports), the package building infrastructure needs to build 2 packages: py-pillow with python2 and py-pillow with python3. Since the ports tree doesn't yet have VARIANTS support, this is not yet possible without making a separate py3-pillow port.
• Ensure graphics/py-pillow does not clobber USES and TKINTER_xxx_DEPENDS variables in slave port
• Remove duplicate USES= line in graphics/py-pillow

PR: 207040
Approved by: mat, adamw (mentors), koobs (maintainer)
Differential Revision: https://reviews.freebsd.org/D6433