math/py-pycosat: Update to 0.6.1

• Update PORTVERSION and distinfo checksum (0.6.1) [1]
• Use PYTHON_SITELIBDIR in post-install STRIP_CMD
• Update test target to new convention

Changes:

https://github.com/ContinuumIO/pycosat/blob/0.6.1/CHANGELOG

PR: 205564 [1]
Submitted by: Yuri <yuri rawbw com> [1]