[db-dtypes] Support pandas 3.1 ExtensionArray compliance interface changes

[chalmerlowe]

Summary

When testing with pandas >= 3.1.0 (currently in pre-release on PyPI as pandas==3.1.0rc0), the db-dtypes compliance test suite (tests/compliance) fails due to upstream changes in pandas internal utilities and ExtensionArray interface contracts.

To prevent presubmit PR CI from failing unexpectedly on developer branches, a temporary ceiling constraint (pandas < 3.1.0dev0) was added in packages/db-dtypes/testing/constraints-3.14.txt. This issue tracks the feature work required to update db-dtypes for full pandas 3.1 compatibility.

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Problem Details

1. Private Import Removal (np_version_gt2)

The compliance test files (test_date_compliance.py, test_json_compliance.py, test_time_compliance.py) currently import a private helper:

from pandas.compat.numpy import np_version_gt2

In pandas 3.1, this private attribute has been removed from pandas.compat.numpy, resulting in test collection errors:

ImportError: cannot import name 'np_version_gt2' from 'pandas.compat.numpy'

2. ExtensionArray Interface Contract Changes

Pandas 3.1 introduces stricter interface requirements in pandas.tests.extension.base:

• fillna mapping: ExtensionArray.fillna() no longer accepts dictionary or mapping values.
• Comparison Operator Outputs: Changes to boolean comparison return types and PyArrow-backed boolean handling.
• Datetime / ZoneInfo fixture dependencies: Test fixtures generating localized timestamps require explicit tzdata resolution on newer Python runtimes.

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Desired Solution

1. Decouple Version Checks:
   Replace from pandas.compat.numpy import np_version_gt2 in tests/compliance/ with standard version checks using packaging.version.parse(np.__version__) >= Version("2.0.0").
2. Update Array Implementations for Pandas 3.1:
   Audit and update DateArray, TimeArray, and JSONArray in db_dtypes/dtypes.py to satisfy both pandas 3.0 and pandas 3.1 ExtensionArray contracts simultaneously.
3. Verify Pre-release & Compliance:
   Run both unit and compliance test suites against pandas>=3.1.0rc0 using nox -s unit_prerelease to ensure 100% compliance.
4. Remove Temporary Constraint:
   Remove pandas < 3.1.0dev0 from packages/db-dtypes/testing/constraints-3.14.txt.