Tests: Adjust expected exception message for Python 3.14.0a6

[hroncok]

E       AssertionError: Regex pattern did not match.
E        Regex: '3 cannot cast to typing.Union\\[str, int\\]'
E        Input: '3 cannot cast to str | int'

Change caused likely by python/cpython@dc6d66f

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