Drop Python 3.12 and raise dependency floors per SPEC 0 (2026 Q4) #8317

Description:

SPEC 0 recommends dropping Python 3.12 and several core dependency versions in 2026 Q4. This issue tracks that bump for scikit-image and the code cleanups it unlocks.

Raise Python floor to 3.13

• pyproject.toml: requires-python = '>=3.13'
• Remove the Programming Language :: Python :: 3.12 classifier
• Drop 3.12 from CI matrices and spin / tooling config

Dependency floors (SPEC 0 2026 Q4)

• Update the pins in pyproject.toml and regenerate requirements/*.txt with tools/generate_requirements.py
• With numpy>=2.3, remove any np.__version__ guards for pre-2.3 behavior

Code cleanups unlocked by the 3.13 floor

• src/_skimage2/_shared/utils.py (~L722): drop the sys.version_info[:2] < (3, 13) branch; keep the un-indented .. deprecated:: note
• src/_skimage2/measure/_regionprops.py (~L1481): keep only the >= (3, 13) doc-parse regex; drop the fallback
• tests/skimage2/_shared/test_utils.py (~L227): inline the inspect.cleandoc-adjusted expected docstring; drop the version check
• src/_skimage2/_shared/utils.py (~L794): remove the sys.version < '3' Python 2 branch

Follow-up: adopt new stdlib

• Replace the function-level path in deprecate_func (src/_skimage2/_shared/utils.py) with the stdlib warnings.deprecated decorator (PEP 702). Example:

from warnings import deprecated

@deprecated("Use Spam instead")
class Ham: ...