Pass fixture scope as a keyword argument

[francesco-ballarin]

Update fixture dectorators according to the changes in pytest-dev/pytest#5776

Fixes warnings like

pytest/5.3.1/lib/python3.6/site-packages/_pytest/fixtures.py:1138: PytestDeprecationWarning: Passing arguments to pytest.fixture() as positional arguments is deprecated - pass them as a keyword argument instead.
  name=name

[francesco-ballarin]

As of pytest 6.0 passing arguments to pytest.fixture() results in an error, see pytest-dev/pytest@c747dc5.

@vtitor is it possible to merge this PR, and push a new release on pypi?

[francesco-ballarin]

The commit mentioned in my previous post has now been included in a release (pytest 6.1.0). The changes in this PR are now mandatory to have pytest-gc run with the current pytest.