Support Python 3.15 in 0.5.0 #459

Python 3.15 is out, and the single-version workflows (lint, coverage, docs, release) already run on it since #458. The package does not declare it yet:

• Add the Programming Language :: Python :: 3.15 classifier.
• Add 3.15 to the Test workflow matrix, the tox env_list and [gh-actions].
• Update the version range in docs/DEVELOPMENT.md.
• Add a changelog fragment.

The full suite already passes on 3.15 locally (1202 passed, 192 skipped), with filterwarnings = error and the locked dependencies.