Create new port graphics/py3-pygraphviz:

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

PR: 207039
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D6281