Personally, I have completely moved away from using pip-requirements.txt files.
These days, I just use setup.py files and simply use pip install -e . to achieve a similar outcome – this has the added benefit of playing very nicely with tox, as well as being fairly consistent and uniform. It even works well with extras_require, so you can additionally install test only deps (when desired) with pip install -e .[tests] or whatever.
pip install -e .
pip install -e .[tests]