Fix for tests: Python 3.13.0a6 renamed pathmod to parser

[befeleme]

This fixes https://github.com/pypa/virtualenv/issues/2671 which reappeared in alpha6 as a result of upstream renaming the attribute.

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)

• [x] ran the linter to address style issues (tox -e fix)
• [x] wrote descriptive pull request text
• [ ] ensured there are test(s) validating the fix
• [ ] added news fragment in docs/changelog folder
• [ ] updated/extended the documentation

[befeleme]

I see 4 failing checks (2x macos, ubuntu and docs), none can be attributed to these changes, windows tests pass. I don't know what to address.