devel/py-dateutil: Update to 2.8.1 [1]

• Remove dateutil_test_test__tz patch, upstreamed [1]
• Remove unecessary pytest-cov dependency
• Add pytest -W exclusion preventing PytestUnknownMarkWarning: Unknown pytest.mark.no_cover error on test invocation.

Changelog:

https://dateutil.readthedocs.io/en/stable/changelog.html#version-2-8-1-2019-11-03

PR: 246104
Submitted by: dvl [1]
MFH: 2020Q2 (blanket: bugfix release)