Tests fail with pytest 4.1

[timhoffm]

Bug report

pytest 4.1 has just been released. According to the changelog, it removes

pytest-dev/pytest#4546: Remove Node.get_marker(name) the return value was not usable for more than a existence check.

which causes our CI tests to fail:

______________________________ test_parse_header _______________________________
[gw1] linux -- Python 3.6.7 /home/travis/virtualenv/python3.6.7/bin/python
self = <pytest_cov.plugin.CovPlugin object at 0x7f551435ca58>
item = <Function test_parse_header>
    @compat.hookwrapper
    def pytest_runtest_call(self, item):
>       if (item.get_marker('no_cover')
                or 'no_cover' in getattr(item, 'fixturenames', ())):
E               AttributeError: 'Function' object has no attribute 'get_marker'
../../../virtualenv/python3.6.7/lib/python3.6/site-packages/pytest_cov/plugin.py:289: AttributeError

This is an issue with pytest-cov: pytest-dev/pytest-cov#253

[timhoffm]

Closed by #13122.