Most of the other answers above don't work with the current version of pip's API. Here is the correct* way to do it with the current version of pip (6.0.8 at the time of writing, also worked in 7.1.2. You can check your version with pip -V).
from pip.req import parse_requirements
from pip.download import PipSession
install_reqs = parse_requirements(<requirements_path>, session=PipSession())
reqs = [str(ir.req) for ir in install_reqs]
setup(
...
install_requires=reqs
....
)
* Correct, in that it is the way to use parse_requirements with the current pip. It still probably isn't the best way to do it, since, as posters above said, pip doesn't really maintain an API.