devel/py-attrs: Update to 18.1.0

[loader]

Summary

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 <[e-mail]> (maintainer)
Differential_Revision: D15923

Test Plan

• portlint: OK (

WARN: Makefile: you may want directory for dependency ${PYTHON_PKGNAMEPREFIX}coverage>0 to be devel/py-coverage:@${FLAVOR}
WARN: Makefile: you may want directory for dependency ${PYTHON_PKGNAMEPREFIX}hypothesis>0 to be devel/py-hypothesis:@${FLAVOR}
WARN: Makefile: you may want directory for dependency ${PYTHON_PKGNAMEPREFIX}Pympler>0 to be devel/py-pympler:@${FLAVOR}
WARN: Makefile: you may want directory for dependency ${PYTHON_PKGNAMEPREFIX}pytest>0 to be devel/py-pytest:@${FLAVOR}
WARN: Makefile: you may want directory for dependency ${PYTHON_PKGNAMEPREFIX}six>0 to be devel/py-six:@${FLAVOR}
WARN: Makefile: you may want directory for dependency ${PYTHON_PKGNAMEPREFIX}zope.interface>0 to be devel/py-zope.interface:@${FLAVOR}
0 fatal errors and 6 warnings found.
)

• testport: OK (poudriere: 1200065, [armv7, aarch64], '', [py27, py36] tested)
• unittest: OK (

Python 2.7: 9 failed, 336 passed, 7 skipped
Python 3.6: 1 failed, 414 passed, 1 skipped
)

[loader]

Thanks @koobs.