pytest: robust file path of a txt file used in tests

During a pytest fixture, what is the best way to robustly get the location of a text file for users that may specify different working directories at runtime e.g. I want a person using the cmd line in the test fixture directory find the file as well as an integration server which may work in the project's root. Can I somehow include the text file in a module? What are best practices for including and getting access to non .py files?

I am aware of BASE_DIR = os.path.dirname(os.path.dirname(__file__)), but I am not sure if this will always refer to the same directory given a particular way of running the test suite.