Addresses pytest 5.1.0 failures

[IAlibay]

Fixes #2329

Just creating this PR ahead of responses to #2329, if raising the pytest version is not desired at this point, I don't mind deleting the PR.

Changes made in this Pull Request:

• Raised the minimum pytest version to 3.3.0 from 3.1.2.
• Changed the match_expr keyword to match in pytest.warns which was causing issues with pytest 5.1.0 (see Tests failing with pytest 5.1.0 #2329 ).

PR Checklist

• Tests? - Fixes existing test
• Docs? - No docs associated as far as I am aware
• CHANGELOG updated?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #2330 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2330      +/-   ##
===========================================
+ Coverage    89.82%   89.82%   +<.01%     
===========================================
  Files          173      173              
  Lines        21711    21711              
  Branches      2846     2846              
===========================================
+ Hits         19501    19502       +1     
  Misses        1615     1615              
+ Partials       595      594       -1

Impacted Files	Coverage Δ
package/MDAnalysis/lib/transformations.py	78.62% <0%> (+0.14%)	⬆️

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[richardjgowers]

merging to get the CI train rolling again, 🚂 thanks @IAlibay