tests: use unittest.mock with py34+

[blueyed]

Fixes #3965.

[codecov]

Codecov Report

Merging #3967 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3967      +/-   ##
==========================================
+ Coverage   94.48%   94.52%   +0.03%     
==========================================
  Files         107      107              
  Lines       23651    23654       +3     
  Branches     2349     2349              
==========================================
+ Hits        22347    22359      +12     
+ Misses        993      988       -5     
+ Partials      311      307       -4

Flag	Coverage Δ
#doctesting	29.31% <ø> (+0.84%)	⬆️
#linux	94.39% <100%> (+0.03%)	⬆️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.23% <ø> (+0.08%)	⬆️
#pexpect	0% <ø> (ø)	⬆️
#pluggymaster	?
#py27	92.63% <60%> (+0.03%)	⬆️
#py34	92.11% <100%> (-0.04%)	⬇️
#py35	92.12% <100%> (-0.04%)	⬇️
#py36	92.7% <100%> (-0.03%)	⬇️
#py37	92.33% <100%> (-0.04%)	⬇️
#trial	31.29% <ø> (+0.06%)	⬆️
#windows	93.85% <100%> (+0.03%)	⬆️
#xdist	18.48% <ø> (+0.02%)	⬆️

Impacted Files	Coverage Δ
testing/code/test_code.py	94.11% <100%> (+0.13%)	⬆️
testing/python/integration.py	91.48% <0%> (-2.13%)	⬇️
testing/test_mark.py	97.05% <0%> (+0.45%)	⬆️
src/_pytest/terminal.py	91.6% <0%> (+1.74%)	⬆️

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[coveralls]

Coverage decreased (-0.05%) to 93.835% when pulling 7537e94 on blueyed:optional-mock into 2c90b3d on pytest-dev:master.

[blueyed]

Meh, running into tox-dev/tox#706 now.

[blueyed]

Squashed, should be good now - although the workaround is a bit unfortunate to have in the first place.