Update deprecated pytest hook arg

[glatterf42]

Our whole CI is failing (same for several downstream projects) because one of the pytest hooks here is using a deprecated argument: startdir has been replaced by start_path. This was kindly pointed out in https://github.com/pytest-dev/pytest/issues/12061, though I have to admit even with the linked deprecation changelog, this might not have been entirely clear to me. After all, the changelog says the hook is now receiving 'additional arguments', not 'changed arguments'. Either way, this PR includes the fix.

Should we update the release notes since other projects might have seen failures due to the hook arguments being out of date? If so, we haven't received an issue yet.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅
• ~[ ] Add or expand tests; coverage checks both ✅~ Just an internal change
• ~[ ] Add, expand, or update documentation.~ Just an internal change
• ~[ ] Update release notes.~ Just an internal change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.9%. Comparing base (049689a) to head (ba6d7af).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #519 +/- ## ===================================== Coverage 98.9% 98.9% ===================================== Files 44 44 Lines 4795 4795 ===================================== Hits 4743 4743 Misses 52 52 ``` | [Files](https://app.codecov.io/gh/iiasa/ixmp/pull/519?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa) | Coverage Δ | | |---|---|---| | [ixmp/testing/\_\_init\_\_.py](https://app.codecov.io/gh/iiasa/ixmp/pull/519?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iiasa#diff-aXhtcC90ZXN0aW5nL19faW5pdF9fLnB5) | `95.9% <100.0%> (ø)` | |