Remove spurious `pytest.warns()` to fix pytest-8 compatibility

[mgorny]

Remove the spurious pytest.warns() contexts within pytest.raises() in test_shortcuts, in order to fix compatibility with pytest-8.0.0. Prior to this version, the exception raised caused these assertions to be ignored entirely. This is fixed in pytest-8.0.0, and the tests start failing because the warning is never raised prior to the exception.

Fixes #789

[codecov[bot]]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (c267743) 90.58% compared to head (36da765) 90.37%. Report is 22 commits behind head on master.

Files	Patch %	Lines
openapi_core/validation/schemas/validators.py	76.47%	2 Missing and 2 partials :warning:
...penapi_core/unmarshalling/schemas/unmarshallers.py	90.00%	2 Missing :warning:
openapi_core/contrib/fastapi/requests.py	80.00%	1 Missing :warning:
openapi_core/contrib/fastapi/responses.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #790 +/- ## ========================================== - Coverage 90.58% 90.37% -0.22% ========================================== Files 139 143 +4 Lines 4207 4228 +21 Branches 514 515 +1 ========================================== + Hits 3811 3821 +10 - Misses 301 311 +10 - Partials 95 96 +1 ```

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[p1c2u]

Thanks for the fix

[mgorny]

Thanks!