Fix call-signature related own PendingDeprecationWarning in tests

[xflr6]

E.g. in https://github.com/xflr6/graphviz/actions/runs/8317979263

PendingDeprecationWarning: The signature of save will be reduced to 1 positional arg ['filename']:
pass directory='doctest-output/round-table.gv' as keyword arg(s)
    filepath = self.save(filename, directory=directory, skip_existing=None)

Possibly related to e5578d3.

[mtelka]

To suppress warnings it is recommended to test with -W ignore:"The signature of":PendingDeprecationWarning.

[kloczek]

Is it not possible to fix that warning instead calming it down? 🤔 (just asking)

[mtelka]

Sure, it is and I believe it will be fixed by @xflr6 soon.

[xflr6]

Sorry for the warning noise (my bad), new relase is underway.

[xflr6]

Fixed in https://pypi.org/project/graphviz/0.20.3/

• reverts in that release: 195855c and 8d8c659

Gives us some time to work on a proper fix.

[xflr6]

Should be fixed by f029709