Replace manual path construction with "os.path.join"

Description of issue or feature request: I found a couple of places in the tests where we have used f strings and manually joined paths instead of using os.path.join(). This can lead to a problem where on Linux and Mac OS this behavior will work, but on Windows won’t. Some examples:

• https://github.com/theupdateframework/python-tuf/blob/e7037cf8c42022ea20f3b7891a50aa3ed563521b/tests/test_api.py#L236
• https://github.com/theupdateframework/python-tuf/blob/e7037cf8c42022ea20f3b7891a50aa3ed563521b/tests/test_api.py#L162

When discussing with @jku why don’t we get CI failures from this behavior he found out that:

File I/O in the Windows API converts forward slashes (“/”) to backslashes (''), before passing the request to the native API

Even though that’s the case, it’s better if we replace those instances of manual path construction with the preferred os.path.join. We should not forget we are developing python-tuf for Windows users as well. 😃

[aribasch]

Hi! New to open source and looking to contribute. I’ve been able to follow the development installation instructions and run tests for python-tuf, and am looking to tackle some good first issues and familiarize myself with the codebase. I was wondering if I would be able to claim this issue?

[jku]

Is there a reason os.path is used throughout the codebase, instead of pathlib?

I believe historical reasons only: pathlib is a fairly new API. I’m ok with using Pathlib, it’s definitely convenient.

I’d argue that it’s only clearer to people who already know Pathlib. E.g. from your examples:

self.repo_dir / "metadata/root.json"

… I admit I would have expected that to create something surprising on windows but maybe it just works?