Not OP, but in our case, there was a package that had a dependency for python3.6 but not for python3.8.

Our production environment was python3.6. Devs rebuilt the requirements.txt with python3.8.

When we attempted to use the requirements.txt with python3.6, we couldn't because a package was missing (and we installed with `--require-hashes`). The dependency was `importlib-metadata` iirc.

But googling around, here's an example of a package that has dependencies that changed based on the python version: https://github.com/pypa/pep517/blob/main/pyproject.toml#L13 .

In our case, we just made sure to rebuild the requirements.txt with the version that matched our production; not sure if there's a "nice" way to support multiple versions with pip-tools.