polygonize: fix flake8 F401, E127, and isort drift

[brendancol]

Closes #2534.

Summary

• Drop the unused is_cupy_array import from .utils (F401). The symbol is not referenced anywhere in polygonize.py and the module has no __all__, so it was never re-exported either.
• Collapse the now-3-symbol .utils import to a single line. At 78 chars it fits within the configured 100-char line_length, so isort prefers the single-line form — same edit closes the isort diff.
• Pull the second argument of the two overload(...) calls in generated_jit back four columns so the continuation aligns with the opening paren (E127).

Categories addressed

• Cat 1 (E127): 2 occurrences
• Cat 3 (F401): 1 occurrence
• Cat 4 (isort): import block reflow

No Cat 2 findings. Cat 5 grep clean (no bare except, no mutable default args, no shadowed builtins).

Backend coverage

Style-only. No behavioural change. The fixes apply uniformly across the numpy / cupy / dask backends dispatched via ArrayTypeFunctionMapping.

Test plan

• flake8 xrspatial/polygonize.py is silent
• isort --check-only --diff xrspatial/polygonize.py is silent
• pytest xrspatial/tests/test_polygonize*.py -- 237 passed, 13 skipped

[brendancol]

Review pass on the head of this branch — nothing to address.

Scope check:

• Diff is 3 lines changed in xrspatial/polygonize.py plus a one-line CSV bookkeeping entry. Pure style sweep, no behavioural delta.

Verified:

• is_cupy_array is not referenced in polygonize.py and the module has no __all__, so the F401 drop is safe. xrspatial/__init__.py and accessor.py only re-export polygonize from this module.
• The collapsed .utils import is 78 chars, well inside the 100-char line_length, matching isort's preferred single-line form.
• The E127 re-indent inside generated_jit only touches whitespace on the strict=False continuation of the two overload(...) calls. No semantic change.

Local checks on this branch:

• flake8 xrspatial/polygonize.py — silent
• isort --check-only --diff xrspatial/polygonize.py — silent
• pytest xrspatial/tests/test_polygonize*.py — 237 passed, 13 skipped

No blockers, suggestions, or nits. No follow-up commit needed.