is this legit in setup.py?

setup.py

import sys

try:
    from setuptools import setup
except ImportError:
    from distutils.core import setup
if 'develop' in sys.argv:
    requirements = [i.strip() for i in open("requirements_dev.txt").readlines()]
else:
    requirements = [i.strip() for i in open("requirements.txt").readlines()]
    
setup(
     # ...
    install_requires=requirements,
  )

extras_require = {
    'develop':  [i.strip() for i in open("requirements_dev.txt").readlines()],
}

Then pip install package[develop] should install it with the additional requirements.
(where package is the name of your package)