gh-105540: Improve relative paths in generate_cases.py

[sobolevn]

In the original code before c413207#diff-65feee563fa44b472b07751184c6f557699f84b3c2ef79174bfd5d538d748272 there were three places where we had os.path.relpath calls. I've changed them to be just prettify_filename calls.

Now I am adding os.path.relpath back to all these three places by including it in prettify_filename.

Plus, changing a comment that is a bit misleading.

• Issue: cases_generator tests aren't run by CI, and are broken #105540

[gvanrossum]

I'm sorry, I don't think I want to do this. It would mean that if the script is run from a directory other than the repo root, errors in the input will be reported incorrectly, and that may confuse users (or even editors).

We only need the relative path in the generated output files, which is exactly where they were before this PR (and after mine).

[sobolevn]

We only need the relative path in the generated output files, which is exactly where they were before this PR (and after mine).

Oh, ok 👍

I was restoring the original behavior from pre-#106713 where we had 3 places where os.path.relpath was used:

1. cpython/Tools/cases_generator/generate_cases.py

   Line 159 in cc25ca1

   filename = os.path.relpath(self.stream.name, ROOT)

2. cpython/Tools/cases_generator/generate_cases.py

   Line 740 in cc25ca1

   filename = os.path.relpath(filename, ROOT)

3. cpython/Tools/cases_generator/generate_cases.py

   Line 1152 in cc25ca1

   os.path.relpath(filename, ROOT).replace(os.path.sep, posixpath.sep)

Please, feel free to close :)

[gvanrossum]

I think we accidentally stumbled upon a better way. :-)