Adopt PEP 639 license metadata

Add license-files. The license field was already the SPDX expression BSD-2-Clause.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Technologicat

pyproject.toml

@@ -8,6 +8,7 @@ dynamic = ["version"]
 description = "Fast and accurate weighted least squares meshless interpolator for Python"
 readme = "README.md"
 license = "BSD-2-Clause"
+license-files = ["LICENSE.md"]
 requires-python = ">=3.11"
 authors = [
     {name = "Juha Jeronen", email = "juha.jeronen@jamk.fi"},