devel/py-buildbot-*: Limit to Python 3.5+

These packages only support Python 3.x [1], currently failing to build:

File "/usr/local/lib/python2.7/site-packages/buildbot_pkg.py", line 254, in run
  super().run()

TypeError: super() takes at least 1 argument (0 given)

antoine@ fixed fallout in py-buildbot-www in ports r500102

[1] https://github.com/buildbot/buildbot/commit/403722c20e7f96707be3d15db54b25f472b8a525

Reported by: pkg-fallout
MFH: 2019Q2 (MFH with 500101, 500102)