chore(core): Update matrix: add pypy3, drop py27

[ceache]

Why is this needed?

By dropping py2.7 and pypy, we drop the last of the py2.x syntax from our tests matrix

Proposed Changes

• Tests are now only run only on py37, py38 and pypy3.
• All py versions run all tests with gevent and sasl.
• Bumped Zookeeper to 3.5.8.
• Update .travis.yml definition per warnings, only syntax refresh.

[ceache]

@StephenSorriaux addressed your comments and also re-added eventlet