https://travis-ci.org/radiasoft/pykern/builds/310874597#L681

pytest version 3.3.0 may have broken capsys with logging
File "/home/travis/build/radiasoft/pykern/tests/pkdebug_test.py", line 115, in test_logging
    assert 'WARNING:root:warn_xyzzy\n' == err, \
AssertionError: When logging is first initialized, warn should output
    assert 'WARNING:root:warn_xyzzy\n' == ''

requirements.txt

@@ -3,7 +3,8 @@ future>=0.14
 jinja2>=2.7
 py-cpuinfo>=0.2
 py>=1.4
-pytest>=2.7
+# capsys may not be working right in 3.3
+pytest>=2.7,<=3.2.3
 pytest-forked>=0.2
 pytz>=2015.4
 pyyaml>=3.0