Bump to Python 3.6 + 3.7

[suzil]

• thriftpy is no longer maintained; thriftpy2 is the active implementation
• test_zmq_message_bus() sometimes fails, so I made the test flaky so that it will try to rerun the test one time if the first try fails

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@02326d2). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #379   +/-   ##
=========================================
  Coverage          ?   60.26%           
=========================================
  Files             ?       78           
  Lines             ?     6145           
  Branches          ?      809           
=========================================
  Hits              ?     3703           
  Misses            ?     2267           
  Partials          ?      175

Impacted Files	Coverage Δ
frontera/contrib/backends/hbase/utils.py	68.75% <100%> (ø)

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[sibiryakov]

Hey, thank you for that. I think it also makes sense to add thriftpy2 to hbase section, so the ones who will be using frontera with hbase will get the right package. Have you checked the compatibility of happybase with thriftpy2 ?

[suzil]

thriftpy2 is fully compatible with thriftpy according to their docs (and they are in the same organization), so it should be OK. I have included thriftpy2 additionally as a requirement of hbase.

[sibiryakov]

okay! merging