Remove Python 3.5 support

[jodal]

Python 3.5 was released in September 2015, thus it does not reach end-of-life before September 2020.

My main motivation for removing support for Python 3.5 before it reaches end-of-life is to be able to use some of the typing improvements in Python 3.6, e.g. typing.NamedTuple and PEP 526 syntax for variable type annotations.

[codecov]

Codecov Report

Merging #89 into develop will increase coverage by 0.18%.
The diff coverage is 94.11%.

@@             Coverage Diff             @@
##           develop      #89      +/-   ##
===========================================
+ Coverage    94.91%   95.09%   +0.18%     
===========================================
  Files           12       12              
  Lines          551      551              
===========================================
+ Hits           523      524       +1     
+ Misses          28       27       -1

Impacted Files	Coverage Δ
pykka/_future.py	100% <ø> (ø)	⬆️
pykka/debug.py	0% <0%> (ø)	⬆️
pykka/_registry.py	100% <100%> (ø)	⬆️
pykka/_proxy.py	98.96% <100%> (ø)	⬆️
pykka/_ref.py	100% <100%> (ø)	⬆️
pykka/_actor.py	95.96% <100%> (+0.8%)	⬆️
pykka/_envelope.py	100% <100%> (ø)	⬆️
pykka/_threading.py	95.65% <100%> (ø)	⬆️

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