Pin affine<3 until pyflwdir supports affine 3

[Zhonghao Zhang (Zhonghao1995)]

Closes #503.

Problem observed

On a fresh pip install -e . of main (f0c18be) on 2026-09-04, macOS 26.6.2, Python 3.11.15, the resolver picks affine 3.0.1, and every run then fails during clip_to_catchments:

numba.core.errors.TypingError: Failed in nopython mode pipeline (step: nopython frontend)
...
- argument 3: Cannot determine Numba type of <class 'affine.Affine'>

affine 3.0 turned Affine from a namedtuple into a plain class. pyflwdir.dem.slope is @njit-compiled and receives the raster transform from geospatial_utilities; numba could type the old tuple and cannot type the new class. pyflwdir 0.5.12, the latest release, has not adapted yet. Installed alongside: pyflwdir 0.5.12, numba 0.67.0, rasterio 1.4.4, numpy 2.4.6.

Change

Pin affine<3 in pyproject.toml. With affine 2.4.0 the same environment builds the documentation's Andorra bounding box end to end (together with #502).

The alternative is to pass a plain tuple to pyflwdir.dem.slope in geospatial_utilities (the numba kernel only indexes transform[0], [4] and [5]); happy to switch to that if you prefer a code change over a pin.

[Zhonghao Zhang (Zhonghao1995)]

Folded into #505: the notebook runs in CI need this pin and the Overture fix at the same time, so keeping them in one PR is the only way to get a green run. Closing this one.