TST: Blacklist pytest versions 3.7.0 and 3.7.1.

This prevents the pytest recursion bug described in
pytest-dev/pytest#3771. For a discussion, see also
pull request odlgroup#1416.

Author: sbanert

test_requirements.txt

@@ -1,5 +1,5 @@
-pytest >= 3.0.3
+pytest >= 3.0.3, != 3.7.0, != 3.7.1
 pytest-pep8
 pytest-cov
 coverage >= 4.0
-coveralls
+coveralls