There is a new technique (Since version 19.1) called Direct references.
Just pretend like your file is hosted on localhost.
from setuptools import setup
path_to_my_project = "/home/user/projects/my_package" # Do any sort of fancy resolving of the path here if you need to
setup(# ... other arguments
install_requires=[f"my_package @ file://localhost/{path_to_my_project}#egg=my_package"]
)