Tests failing with pytest 5.1.0

[IAlibay]

Expected behavior

Issue with pytest 5.1.0 (released 15th August 2019)
pytest test_util.py in /lib should not fail.

Actual behavior

When using pytest 5.1.0, test_deprecate fails due to the match_expr keyword. This can be seen in https://travis-ci.com/MDAnalysis/mdanalysis/jobs/226110046

Conda currently provides pytest 5.0.1, so all the non-MacOS builds are not failing. But the py3.6 MacOS build installs pytest via pip, which picks up 5.1.0. I have tested 5.1.0 locally on a linux machine and the failures happen too.

Possible fix

It looks like the match_expr keyword was removed in: pytest-dev/pytest@683b263 (and pytest-dev/pytest#5529)
If match_expr is replaced with match the tests will run normally.

I'm not 100% sure when match_expr first got deprecated though. It looks like it was ~ 2 years ago, so somewhere around pytest 3.X?

Iterating through older versions of pytest, it looks like the minimum version where the match keyword will work is 3.3.0. I should note that in pytest versions prior to that, changing the value of match_expr to something that should fail doesn't actually cause test failures either (tested 3.1, 3.2.0 and 3.2.5).

If it's ok I'll create a PR switching match_expr to match and bump up the minimum version of pytest to 3.3.0?

Currently version of MDAnalysis

• Which version are you using? (run python -c "import MDAnalysis as mda; print(mda.__version__)")
  0.19.3-dev
• Which version of Python (python -V)?
  Python 3.7.3
• Which operating system?
  Ubuntu 18.04 LTS

[richardjgowers]

So pytest is just used for devs right? (or is the MDAnalysis.test() function a thing which uses pytest?). So can't we be more aggressive with updating?