Release 3.9.1

[RonnyPfannschmidt]

Thanks for submitting a PR, your contribution is really appreciated!

Here's a quick checklist that should be present in PRs (you can delete this text from the final description, this is
just a guideline):

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS in alphabetical order;

[RonnyPfannschmidt]

had to amend

• doc/en/example/parametrize.rst
• doc/en/example/pythoncollection.rst
• doc/en/skipping.rst

for import missmatches

[codecov]

Codecov Report

Merging #4171 into features will decrease coverage by 2.38%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           features    #4171      +/-   ##
============================================
- Coverage     94.02%   91.64%   -2.39%     
============================================
  Files           109      109              
  Lines         24203    24211       +8     
  Branches       2384     2384              
============================================
- Hits          22758    22187     -571     
- Misses         1125     1503     +378     
- Partials        320      521     +201

Flag	Coverage Δ
#linux	91.64% <ø> (-2.28%)	⬇️
#py27	91.64% <ø> (-0.08%)	⬇️
#py34	?
#py36	?
#windows	?
#xdist	?

Impacted Files	Coverage Δ
src/_pytest/compat.py	76.43% <0%> (-20.12%)	⬇️
testing/code/test_excinfo.py	82.59% <0%> (-14.16%)	⬇️
testing/test_paths.py	86.36% <0%> (-13.64%)	⬇️
src/_pytest/reports.py	86.9% <0%> (-10.72%)	⬇️
testing/test_compat.py	79.1% <0%> (-10.45%)	⬇️
testing/test_modimport.py	70% <0%> (-10%)	⬇️
testing/test_pastebin.py	91.66% <0%> (-8.34%)	⬇️
src/_pytest/assertion/rewrite.py	87.56% <0%> (-7.97%)	⬇️
src/_pytest/_code/code.py	88.28% <0%> (-7.1%)	⬇️
src/_pytest/pathlib.py	83.12% <0%> (-5%)	⬇️
... and 54 more

---

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[nicoddemus]

Going ahead and merging this now that it has been published. 👍