Open files test failure on latest build #6942

The latest build on master has a test failure from pytest-openfiles.

I just released a new version pytest-astropy. While there haven't been any substantive changes to pytest-openfiles since the last release, pytest-astropy now includes pytest-mpl as a dependency, which includes matplotlib as a dependency.

The test matrix does not explicitly install matplotlib during the --open-files test run, so I think this test failure is real but it has never been exposed before. It may be that we just need a way to suppress it though if it is not actually coming from Astropy itself.