databases/py-rrdtool: New port, version 0.1.15

As the successor to the deprecated port databases/py-rrdtool_lgpl [1],
this port is intended to inherit its functionality, but comes from a
different source [2][3] and is also named differently.

[1] https://sourceforge.net/projects/py-rrdtool/files/
[2] http://www.nongnu.org/py-rrdtool/
[3] https://github.com/commx/python-rrdtool

PR: 251126
Submitted by: darius@dons.net.au (new maintainer)
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D28038