net/py-wsproto: Fix runtime dependencies and Python version restriction

• Update the RUN_DEPENDS and limit the port to Python 3.6 or newer as the support for Python 2.7 and 3.5 was dropped with the 0.15.0 release.
  
  Python 3.6 needs also the devel/py-dataclasses package as an additional dependency to work properly.

• Add a "do-test" target while I'm here to make future QA easier.

• Bump PORTREVISION for package change.

PR: 242025 (related)
Approved by: portmgr blanket (runtime fix)
MFH: 2019Q4 (runtime fix)