While debugging this I found a useful clue in https://github.com/pypa/pip/blob/e060970d51c5946beac8447eb95585d83019582d/src/pip/_internal/cli/main.py#L23-L47

```

Do not import and use main() directly! Using it directly is actively

discouraged by pip's maintainers. The name, location and behavior of

this function is subject to change, so calling it directly is not

portable across different pip versions.

In addition, running pip in-process is unsupported and unsafe. This is

elaborated in detail at

https://pip.pypa.io/en/stable/user_guide/#using-pip-from-your-program.

That document also provides suggestions that should work for nearly

all users that are considering importing and using main() directly.

However, we know that certain users will still want to invoke pip

in-process. If you understand and accept the implications of using pip

in an unsupported manner, the best approach is to use runpy to avoid

depending on the exact location of this entry point.

The following example shows how to use runpy to invoke pip in that

case:

sys.argv = ["pip", your, args, here]

runpy.run_module("pip", run_name="main")

Note that this will exit the process after running, unlike a direct

call to main. As it is not safe to do any processing after calling

main, this should not be an issue in practice.

```