textproc/py-rdflib: Make concurrent safe

This package installs scripts into LOCALBASE/bin, so must be concurrent
safe so packages for multiple Python versions don't conflict [1] on
installation.

[1] https://lists.freebsd.org/pipermail/freebsd-ports/2019-May/116293.html

Reported by: Luis Espinoza Jr. <ljejr hotmail com> (via freebsd-ports) [1]
Approved by: koobs (python, maintainer)
MFH: 2019Q2