There's pip-compile from https://pypi.org/project/pip-tools/ that does exactly that. Pipenv uses its resolving mechanism if I'm not mistaken. It produces standard req.txt file with versions pinned and supports separate dev requirements. It had some bug with upgrades last time I checked though, not sure whether it's resolved, currently considering using it for projects at work.
Since its all just requirements files with pip-tools, its been fairly commitment free, in that I haven't had to substantially rework any major bits of workflow to use it - nor would I to remove it. Not sure I could say the same thing about pipenv and/or poetry.
The "library" workflow works for applications too. Put your direct dependencies in setup.py. Build wheels of everything and upload them to an internal PyPI server. Pin everything in requirements.txt.
You wouldn't write your application to support 5 versions of Django, but you _probably_ would do so for a library.
That said, I do basically agree about `pip` existing already. We could have built a tool to manipulate `requirements.txt` files instead of introducing another format and a toolchain that is _much_ slower and brittler. Though ultimately at this point Python packaging woes feel like they are at a much lower level (the fact that libraries end up being "installed" mean that preparing all your dependencies to go out to multiple servers is a mess).