pytest >=6.0 causes pylint errors about not implemented abstract methods

[andy-maier]

https://travis-ci.org/github/pywbem/pywbem/jobs/713389868

On Python 3.8 wirth pytest 6.0.1:

tests/functiontest/conftest.py:225:0: W0223: Method 'get_closest_marker' is abstract in class 'Node' but is not overridden (abstract-method)
tests/functiontest/conftest.py:225:0: W0223: Method 'gethookproxy' is abstract in class 'FSCollector' but is not overridden (abstract-method)
tests/functiontest/conftest.py:225:0: W0223: Method 'isinitpath' is abstract in class 'FSCollector' but is not overridden (abstract-method)
tests/functiontest/conftest.py:252:0: W0223: Method 'get_closest_marker' is abstract in class 'Node' but is not overridden (abstract-method)

pylint version:
pylint 2.4.4
astroid 2.3.3
Python 3.8.0 (default, Oct 15 2019, 17:49:23)

pytest version:
platform linux -- Python 3.8.0, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
plugins: cov-2.10.0, yagot-0.5.0, requests-mock-1.8.0

Reported this as pytest-dev/pytest#7591.