ANN: Please migrate away from pytest-openfiles

[pllim]

Hello. The Astropy Project has decided to retire pytest-openfiles, a plugin to detect open files in pytest runs. We see that you are using it in this repo. While you may continue to use it, please be advised that it will no longer be maintained going forward. See https://github.com/astropy/pytest-openfiles#important-retirement-roadmap for more information. We apologize for any inconvenienced cause.

If this issue was opened in error and is irrelevant, feel free to close.

Thank you for your patience.

xref astropy/pytest-openfiles#47