Merge pull request matplotlib#18606 from hugovk/unpin-pytest

Revert "FIX: pin pytest"

Author: jklymak

requirements/testing/travis_all.txt

@@ -2,7 +2,7 @@
 
 certifi
 coverage
-pytest!=4.6.0,!=5.4.0,!=6.1.0
+pytest!=4.6.0,!=5.4.0
 pytest-cov
 pytest-rerunfailures
 pytest-timeout