security/py-pywinrm: Allow builds with Python 3.x

Upstream supports and tests against up to Python 3.6 [1] and declares up to
3.6 in its Trove Classifiers [2].

Fix incorrect Python version support declaration in USES=python accordingly,
allowing Python 3.x builds. [3]

While I'm here, add TEST_DEPENDS and a test target to help with QA.

[1] https://github.com/diyan/pywinrm/blob/master/.travis.yml
[2] setup.py: 'Programming Language :: Python :: 3.6'

PR: 237178 [3]
Reported by: <timp87 gmail com> [3]
Approved by: portmgr (blanket: bug fix, framework compliance)
MFH: 2019Q2