Merge pull request #1639 from pypa/bugfix/1638-pytest-pin

Pin pytest to <4 for now. Ref #1638.

tests/requirements.txt

@@ -3,7 +3,8 @@ pytest-flake8; python_version!="3.4"
 pytest-flake8<=1.0.0; python_version=="3.4"
 virtualenv>=13.0.0
 pytest-virtualenv>=1.2.7
-pytest>=3.0.2
+# pytest pinned to <4 due to #1638
+pytest>=3.7,<4
 wheel
 coverage>=4.5.1
 pytest-cov>=2.5.1