Pass config file to black, if present

[dcode]

This is similar to #9, but if additional black settings are configured in pyproject.toml beyond the include/exclude directives, it would be nice to have pytest-black honor those. I think this is as simple as adding --config pyproject.toml to the command argv in runtest(). This fixes my issues with line length discrepancy between config and test.

I can submit a PR to address it.

[robertschonfeld]

Hey, I think your issue is not correct. This package does honor the pyproject.toml configuration as it's doesn't prevent the default behaviour of Black to look for this file.

There is the issue #65 which may cause confusion for new users as it did for me.