Pin pytest until we drop 2.6

[hynek]

pytest 3.3 drops Python 2.6 and (ironically) Python 3.3.

I would of course much prefer to drop 2.6 and cryptography 2.2 is dropping 2.6 so an alternative PR to this one would be to kill 2.6 with fire if cryptography 2.2 is on a realistic horizon.

We need to do either of those two because otherwise our tests will fail.

[codecov]

Codecov Report

Merging #721 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #721   +/-   ##
=======================================
  Coverage   97.03%   97.03%           
=======================================
  Files          18       18           
  Lines        5669     5669           
  Branches      394      394           
=======================================
  Hits         5501     5501           
  Misses        112      112           
  Partials       56       56

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[alexwlchan]

I’d much prefer dropping Python 2.6 support, but this is fine for now. 👍

[reaperhulk]

cryptography 2.2 is probably 4-8 weeks away (we haven't really done much work thus far).