Resolve dependency conflict of pytest-cov

test-requirements.txt

@@ -1,7 +1,14 @@
 # Add requirements only needed for your unittests and during development here.
 # They will be installed automatically when running `python setup.py test`.
 # ATTENTION: Don't remove pytest-cov and pytest as they are needed.
-pytest-cov
+# TODO: Remove the pinning of the pytest-cov version again once issue
+#       https://github.com/z4r/python-coveralls/issues/66
+#       is resolved.
+#       Background: pytest-cov 2.6.0 has increased the version
+#       requirement for the coverage package from >=3.7.1 to
+#       >=4.4, which is in conflict with the version requirement
+#       defined by the python-coveralls package for coverage==4.0.3.
+pytest-cov>=2.4.0,<2.6
 pytest
 hypothesis[numpy]
 Sphinx