zonal: fix flake8 F401, E501 + isort drift

[brendancol]

Closes #2522.

Summary

• Drop unused not_implemented_func import (F401, Cat 3).
• Wrap the 117-char dd.concat one-liner at line 455 (E501, Cat 1).
• Consolidate the xrspatial.utils import block — merge the trailing
  has_dask_array, alphabetise, trim the extra blank line above
  # 3rd-party (Cat 4, isort).

No behavioural change. not_implemented_func was only on the import
line; there are no call sites.

Test plan

• flake8 xrspatial/zonal.py clean
• isort --check-only xrspatial/zonal.py clean
• pytest xrspatial/tests/test_zonal.py -x — 125 passed

[brendancol]

PR Review: zonal: fix flake8 F401, E501 + isort drift

Blockers

None.

Suggestions

None.

Nits

None.

What looks good

• Diff matches the three findings in issue #2522: one F401, one E501, one isort consolidation. No drive-by edits.
• not_implemented_func is unused. Grep across the file finds zero call sites, and grep across xrspatial/ finds no re-export consumer (__init__.py and accessor.py import specific functions from xrspatial.zonal, none of them not_implemented_func).
• The dd.concat wrap preserves call semantics: same positional list, same axis=1, ignore_unknown_divisions=True kwargs.
• The retained import block keeps all 9 names used elsewhere in the module (43 references across the file).
• flake8 xrspatial/zonal.py and isort --check-only xrspatial/zonal.py both pass on the PR head.
• 125 zonal tests pass locally on the PR head.

Checklist

• Algorithm matches reference/paper (N/A, no algorithm change)
• All implemented backends produce consistent results (N/A, no dispatch change)
• NaN handling is correct (N/A)
• Edge cases are covered by tests (N/A, existing tests cover behaviour)
• Dask chunk boundaries handled correctly (N/A)
• No premature materialization or unnecessary copies (N/A)
• Benchmark exists or is not needed (N/A, pure style)
• README feature matrix updated (N/A, no API change)
• Docstrings present and accurate (N/A)

[brendancol]

PR Review

Blockers

None.

Suggestions

None.

Nits

None.

What looks good

• Scope matches the description: one F401, one E501 wrap, one isort consolidation.
• not_implemented_func had no call sites in xrspatial/zonal.py; removal is safe and the symbol remains exported from xrspatial.utils for other modules.
• The wrapped dd.concat at lines 451-454 keeps argument order and trailing-comma style consistent with the rest of the file.
• flake8 xrspatial/zonal.py clean, isort --check-only xrspatial/zonal.py clean, all 125 tests in test_zonal.py pass on the branch.

Checklist

• Algorithm matches reference (no algorithm change)
• All implemented backends produce consistent results (no dispatch change)
• NaN handling unchanged
• Edge cases covered by existing tests
• Dask chunk boundaries unchanged
• No premature materialization introduced
• Benchmark not needed (style-only)
• README feature matrix not affected
• Docstrings unchanged

Ready to merge.