Fix failing test for 3.7, extend job matrix on Travis with 3.8 support

[hoefling]

What do these changes do?

This change fixes a remaining test failing with Python 3.7, in particular adapting to changes in https://github.com/python/cpython/pull/4365 same way the SelectorTransport does. In addition, it extends the job matrix on travis-ci.com to test against Python 3.8 and combinations of msgpack extra installed and debug mode for asyncio turned on. Last change is the pyzmq version constraint fixed to allow the recent pyzmq version to be installed.

Are there changes in behavior for the user?

No

Related issue number

Closes #132.

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [ ] Documentation reflects the changes no changes in behaviour
• [ ] Add a new news fragment into the CHANGES folder towncrier not set up

[codecov[bot]]

Codecov Report

Merging #159 into master will increase coverage by 0.06%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   92.41%   92.48%   +0.06%     
==========================================
  Files          14       14              
  Lines        1634     1649      +15     
  Branches      271      274       +3     
==========================================
+ Hits         1510     1525      +15     
  Misses        101      101              
  Partials       23       23

Impacted Files	Coverage Δ
aiozmq/core.py	97.94% <100%> (+0.01%)	:arrow_up:

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[JelleZijlstra]

Thank you!

[hoefling]

@JelleZijlstra gladly! Thank you for a fast reply! Would you have time to review more PRs? In particular, I would like to add type hints so I don't need to maintain my own type stubs, also try to add more tests to achieve 100% line/branch coverage and after that try to move to native coroutines, as requested in #142.

[JelleZijlstra]

Definitely! I'd be happy to accept typing (preferably inline, not as stubs, but I'd take stubs too). I'm planning to release the current state of the repo as 0.9.0 as soon as I get permissions on pypi (asked on #131) so that this library finally supports 3.7 and 3.8, and we can do another release in some time.

[logileifs]

Can this release go out soon? After reading https://github.com/aio-libs/aiozmq/issues/131 it seems like all the permissions are in place :)

[JelleZijlstra]

Done.

[logileifs]

Thanks, great job!