Eventlet event fixes

[jodal]

This fixes inconsistencies between the pykka.eventlet.EventletEvent API and Python 3's threading.Event API.

[codecov]

Codecov Report

Merging #91 into develop will increase coverage by 0.17%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop      #91      +/-   ##
===========================================
+ Coverage    95.16%   95.33%   +0.17%     
===========================================
  Files           12       12              
  Lines          558      557       -1     
===========================================
  Hits           531      531              
+ Misses          27       26       -1

Impacted Files	Coverage Δ
pykka/eventlet.py	90.32% <100%> (+1.43%)	⬆️

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