Remove usage of py.std

[asottile]

Refs #610
Resolves #592

[codecov]

Codecov Report

Merging #621 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #621      +/-   ##
==========================================
+ Coverage   93.63%   93.68%   +0.04%     
==========================================
  Files          11       11              
  Lines        2357     2359       +2     
==========================================
+ Hits         2207     2210       +3     
+ Misses        150      149       -1

Impacted Files	Coverage Δ
tox/venv.py	96.26% <ø> (ø)	⬆️
tox/session.py	94.01% <100%> (+0.01%)	⬆️
tox/_pytestplugin.py	93.97% <100%> (+0.4%)	⬆️
tox/result.py	100% <100%> (ø)	⬆️

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