devel/py-google-pasta: Add missing dependency

six is a documented [1][2] dependency, add it to install_requires
accordingly.

While I'm here, add a test target and patch setup.py so it doesn't run
the tests twice.

[1] https://github.com/google/pasta#dependencies
[2] https://github.com/google/pasta/pull/77

Approved by: portmgr (blanket(s): missing dependencies, framework)
MFH: 2019Q3