Style cleanup: flake8 + isort across xrspatial.geotiff subpackage

[brendancol]

Summary

Closes #2282.

Behaviour-neutral cleanup of every flake8 and isort report against xrspatial/geotiff/. Starting count: 309 flake8 violations, 22 isort-out-of-order files. Final: both tools clean.

Categories addressed

• Cat 1 (E-codes): continuation indents (E127/E128), blank-line spacing (E302/E303/E305), nested-def blank line (E306), module-level-import (E402) with # noqa for pytest.importorskip and late-binding cases, long-line (E501) with # noqa for XML fixtures and codec lookup tables.
• Cat 2 (W391): trimmed trailing blank lines at EOF in four files.
• Cat 3 (F-codes): F401 unused imports deleted; F811 redefinitions cleaned; F841 unused locals removed; F821 cupy annotation fixed via TYPE_CHECKING import; F541 f-string without placeholders converted.
• Cat 4 (isort): re-ordered imports across 60+ files using the project's own isort config (line_length=100).

Behaviour-changing fixes (Cat 3)

Each F841 was a leftover, not a real bug. All are deletions, no logic added:

• _geotags.py geokeys = []: pre-refactor scratch list, the real one is key_entries.
• _gpu_decode.py n_tiles = len(d_tile_bufs): re-bound from local later.
• tests/test_features.py dtype_np, tiles_per_band: scratch locals.
• tests/test_security.py header = parse_header(base): computed but never asserted.
• tests/test_edge_cases.py with open(path, 'wb') as f: f was unused; rebound to bare with open(...).

The F821 cupy undefined-name in _writers/gpu.py:71 is a type annotation only (xr.DataArray | cupy.ndarray | np.ndarray). Fixed by importing cupy under TYPE_CHECKING so pyflakes resolves the name without forcing a runtime import.

Re-export strategy for __init__.py

xrspatial.geotiff/__init__.py re-exports a long list of internal helpers under leading-underscore aliases (_coords_from_pixel_geometry, _transform_tuple, etc) that are part of the documented internal API surface. The existing convention at lines 89 and 119 already uses # noqa: F401. This PR extends the same convention to every other re-export line so pyflakes stops flagging them.

Out of scope

• No flake8 or isort config changes in setup.cfg.
• No autoformatter (black, ruff format, autopep8) was run.
• No functional refactors.

Test plan

• flake8 xrspatial/geotiff/ returns clean.
• isort --check-only xrspatial/geotiff/ returns clean.
• import xrspatial.geotiff succeeds; __all__ unchanged.
• pytest xrspatial/geotiff/tests/test_header.py xrspatial/geotiff/tests/test_geotags.py xrspatial/geotiff/tests/test_edge_cases.py passes (140 tests).
• pytest xrspatial/geotiff/tests/test_features.py -m "not slow" passes (126 passed, 4 skipped).

[brendancol]

Self-review notes for the style cleanup.

Scope check:

• flake8 xrspatial/geotiff/ now exits 0 (was 309 violations).
• isort --check-only xrspatial/geotiff/ now exits 0 (was 22 files out of order).
• setup.cfg flake8/isort config unchanged.

Behaviour-touching changes are limited to:

• F401 unused imports removed (verified each name is not referenced elsewhere in the file).
• F841 unused locals removed (all leftover scratch variables, no assertions or side effects lost).
• F821 cupy annotation fixed by adding cupy under TYPE_CHECKING in _writers/gpu.py. No runtime import added.
• F541 f-string -> plain string in _validation.py:312.
• F811 redefinitions cleaned (one in init.py was a nested import os inside a function; the module already imports os at the top).

Re-export surface preserved: every line in xrspatial.geotiff/init.py that re-exports a _xxx-prefixed internal helper now carries # noqa: F401 so pyflakes does not flag the alias. The existing convention already used this at lines 89 and 119.

E501 noqa was applied to: XML fixture lines in test_features.py and bench_vs_rioxarray.py, GeoTIFF metadata field descriptions in _write_layout.py, and the f'{"f" if ...}' codec dtype string in _vrt.py:1572 that resists clean line breaks.

E402 noqa applied to: imports that sit after a pytest.importorskip (test files) and the late from numba import int32 as numba_int32 set in _gpu_decode.py:317 where the import is intentionally placed below the device kernels that consume it.

Smoke tests:

• test_header.py, test_geotags.py, test_edge_cases.py: 140 passed.
• test_features.py (-m "not slow"): 126 passed, 4 skipped.

The merge with main is currently flagged as conflicting; resolving in Step 11.