re-enable linting tox env on windows

[asottile]

Resolves #4094

[nicoddemus]

Awesome, thanks for that @asottile, appreciate it! 🙇

[RonnyPfannschmidt]

fabulous work 👍

[coveralls]

Coverage increased (+0.02%) to 93.812% when pulling 11a0721 on asottile:re-enable_linting_windows into b8c30aa on pytest-dev:master.

[codecov]

Codecov Report

Merging #4099 into master will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4099      +/-   ##
=========================================
- Coverage    94.5%   94.4%   -0.11%     
=========================================
  Files         109     109              
  Lines       23810   23810              
  Branches     2349    2349              
=========================================
- Hits        22502   22477      -25     
- Misses       1000    1021      +21     
- Partials      308     312       +4

Flag	Coverage Δ
#doctesting	28.47% <ø> (-0.71%)	⬇️
#linux	94.4% <ø> (+0.04%)	⬆️
#nobyte	0% <ø> (ø)	⬆️
#numpy	28.08% <ø> (-0.01%)	⬇️
#pexpect	0% <ø> (ø)	⬆️
#py27	92.56% <ø> (-0.07%)	⬇️
#py34	92% <ø> (-0.09%)	⬇️
#py35	92.01% <ø> (-0.09%)	⬇️
#py36	92.54% <ø> (-0.13%)	⬇️
#py37	92.17% <ø> (-0.14%)	⬇️
#trial	31.16% <ø> (-0.16%)	⬇️
#windows	?
#xdist	18.51% <ø> (-0.05%)	⬇️

Impacted Files	Coverage Δ
src/_pytest/config/__init__.py	92.37% <ø> (ø)	⬆️
testing/test_paths.py	86.36% <0%> (-13.64%)	⬇️
src/_pytest/paths.py	91.3% <0%> (-8.7%)	⬇️
testing/test_tmpdir.py	95.06% <0%> (-4.94%)	⬇️
src/_pytest/capture.py	86.72% <0%> (-3.21%)	⬇️
src/_pytest/nodes.py	93.69% <0%> (-0.85%)	⬇️
testing/acceptance_test.py	97.14% <0%> (-0.66%)	⬇️
src/_pytest/pytester.py	85.56% <0%> (-0.46%)	⬇️
testing/test_capture.py	98.94% <0%> (-0.31%)	⬇️
src/_pytest/fixtures.py	97.02% <0%> (-0.28%)	⬇️
... and 1 more

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