Proposed commit log message:
```
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:
```