upgrade pytest dependency

[emmettbutler]

This pull request fixes #782 by upgrading the pytest dependency and removing the code used to work around pytest-dev/pytest#568.

[codecov-io]

Codecov Report

Merging #839 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #839   +/-   ##
=======================================
  Coverage   83.98%   83.98%           
=======================================
  Files          27       27           
  Lines        3621     3621           
  Branches      547      547           
=======================================
  Hits         3041     3041           
+ Misses        437      435    -2     
- Partials      143      145    +2

Impacted Files	Coverage Δ
pykafka/partition.py	72.72% <0%> (-12.13%)	⬇️
pykafka/topic.py	82.47% <0%> (-2.07%)	⬇️
pykafka/simpleconsumer.py	85.01% <0%> (-0.9%)	⬇️
pykafka/cluster.py	73.03% <0%> (-0.75%)	⬇️
pykafka/handlers.py	93.38% <0%> (ø)	⬆️
pykafka/protocol.py	94.11% <0%> (+0.21%)	⬆️
pykafka/balancedconsumer.py	89.47% <0%> (+0.29%)	⬆️
pykafka/broker.py	92.98% <0%> (+3.5%)	⬆️
pykafka/managedbalancedconsumer.py	84.1% <0%> (+4.63%)	⬆️

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