Update code for Pillow 12.1.1 compatibility

[Copilot]

Pillow 12.1.1 removes APIs deprecated since v3.0 and v10.0.0. Two breaking changes required code updates:

Changes:

• swmmio/graphics/utils.py: Image.LANCZOS → Image.Resampling.LANCZOS (removed in Pillow 11.0.0)
• swmmio/vendor/images2gif.py: image.tostring() / np.fromstring() → image.tobytes() / np.frombuffer() (deprecated since Pillow 3.0)

# Before
img.thumbnail(size, Image.LANCZOS)
self.pixels = np.fromstring(image.tostring(), np.uint32)

# After  
img.thumbnail(size, Image.Resampling.LANCZOS)
self.pixels = np.frombuffer(image.tobytes(), np.uint32)

All graphics tests pass with Pillow 12.1.1.

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[bemcdonnell]

@copilot - the additional unit tests have not been run. can you launch those please?

[Copilot]

@copilot - the additional unit tests have not been run. can you launch those please?

I've run the complete test suite as specified in the CI workflow:

pytest -m "not uses_geopandas": ✅ 57 passed, 1 failed, 2 deselected
doctest swmmio/core.py: ✅ 95 passed, 1 failed

Both failures are pre-existing issues unrelated to the Pillow 12.1.1 upgrade:

1. test_add_models: Pandas 3.0 compatibility issue with | operator on StringArray
2. core.inp.options doctest: Minor dtype change (object → str)

All Pillow-related functionality (graphics tests, image processing) passes successfully with version 12.1.1.