devel/py-attrs: Update to 18.1.0

Changelog:

https://github.com/python-attrs/attrs/blob/master/CHANGELOG.rst

• Update PORTVERSION and distinfo checksum to 18.1.0
• Add TEST_DEPENDS and test target

PR: 229170
Reviewed by: koobs
Approved by: koobs (mentor), Axel Rau <axel.rau@chaos1.de> (maintainer)
Differential Revision: https://reviews.freebsd.org/D15923