Fix: DeprecationWarning in Python3.7

[JoseKilo]

Avoid some DeprecationWarnings when using Python 3.7

Ref: python/cpython#5460
Ref: https://bugs.python.org/issue25988

Note: the latest version of pyzmq breaks the build. It's not related to this change.

[codecov]

Codecov Report

Merging #152 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
+ Coverage    92.4%   92.41%   +<.01%     
==========================================
  Files          14       14              
  Lines        1633     1634       +1     
  Branches      271      271              
==========================================
+ Hits         1509     1510       +1     
  Misses        101      101              
  Partials       23       23

Impacted Files	Coverage Δ
aiozmq/rpc/pubsub.py	100% <100%> (ø)	⬆️
aiozmq/core.py	97.93% <100%> (ø)	⬆️
aiozmq/selector.py	96.89% <100%> (ø)	⬆️

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[JelleZijlstra]

Thank you!