Fix mypy errors with numpy 2.3 type stubs

[max-sixty]

Summary

Fixes the mypy test failures reported in #10791. The issue was caused by numpy 2.3's stricter type stubs revealing legitimate type issues in our codebase.

The Problem

CI environments use numpy 2.3.3 (via conda-forge) while local development was constrained to numpy<2.3 due to an outdated numba compatibility constraint. NumPy 2.3 has stricter type annotations that caught several type issues:

1. In xarray/core/indexing.py: Lists were being used to hold mixed types (slices and ndarrays) without proper type annotations
2. In test files: Various type compatibility issues with MaskedArray, method overrides, and union types

Changes

1. Updated dependency constraints:

   • Removed numpy<2.3 constraint
   • Updated numba>=0.62 (numba 0.62+ supports numpy 2.3)

2. Fixed type annotations:

   • Added proper type hints for backend_indexer_elems and np_indexer_elems in indexing.py
   • These lists hold slice | np.ndarray types for use with OuterIndexer and VectorizedIndexer

3. Added targeted type ignores:

   • test_namedarray.py: MaskedArray not in duckarray union
   • test_parallelcompat.py: Covariant return type in compute method override
   • Fixed assertion in test_dataarray.py to handle union type for mask attribute

Testing

• ✅ All mypy checks pass
• ✅ All affected tests pass
• ✅ Pre-commit hooks pass

Closes #10791

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