Fix plurals in validation error messages (in tests)

[Iipin]

Change Summary

A complementary change to pydantic/pydantic-core#1050 – adjust grammar in error message assertions.

Related issue number

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[sydney-runkle]

Awesome, looks great overall. Thanks for your contribution!

Could you add a pytest.mark.xfail marker on all of the tests that you changed similar to this:

pydantic/tests/test_serialize.py

Line 1120 in f89077a

@pytest.mark.xfail(reason='Needs new release of pydantic-core')

We can remove this once we release a new version of pydantic-core with your changes :)

[Iipin]

@sydney-runkle thank you for the comment – marked failing tests, ptal. I see that pypy3.9 has segfaulted in CI but I don't think this has anything to do with my change so I'm marking the PR as ready :)

[sydney-runkle]

Looks awesome :), thanks for the contribution!