Workaround for editable installs with pip 19.1

[nicoddemus]

Related to #5167

[codecov]

Codecov Report

Merging #5168 into master will decrease coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5168      +/-   ##
==========================================
- Coverage   96.06%   95.79%   -0.28%     
==========================================
  Files         114      114              
  Lines       25825    25825              
  Branches     2550     2550              
==========================================
- Hits        24810    24740      -70     
- Misses        705      757      +52     
- Partials      310      328      +18

Impacted Files	Coverage Δ
testing/test_pathlib.py	91.17% <0%> (-8.83%)	⬇️
testing/test_tmpdir.py	95.16% <0%> (-3.77%)	⬇️
testing/deprecated_test.py	96.77% <0%> (-3.23%)	⬇️
src/_pytest/capture.py	91.9% <0%> (-3.07%)	⬇️
src/_pytest/compat.py	93.96% <0%> (-3.02%)	⬇️
src/_pytest/pathlib.py	88.46% <0%> (-2.2%)	⬇️
src/_pytest/config/findpaths.py	94% <0%> (-1%)	⬇️
testing/acceptance_test.py	97.05% <0%> (-0.99%)	⬇️
src/_pytest/nodes.py	94.6% <0%> (-0.99%)	⬇️
testing/test_parseopt.py	97.16% <0%> (-0.81%)	⬇️
... and 9 more

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[nicoddemus]

If Travis is happy, I think we can get this merged, proves the docs environment is working at least.