The new pytest-cov 2.10.0 requires pytest>=4.6 but pytest 4.3.1 is used for Python < 3.5 which is strange because dev-requirements.txt says pytest>=3.6.0,<5.0.0 ; python_version < '3.5'
See https://api.travis-ci.org/v3/job/700032657/log.txt for example