fix: remove license classifier to comply with PEP 639

When using SPDX license expression, the license classifier
should not be used.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tanbro

pyproject.toml

@@ -32,7 +32,6 @@ license-files = ["LICENSE"]
 classifiers = [
   "Development Status :: 5 - Production/Stable",
   "Intended Audience :: Developers",
-  "License :: OSI Approved :: BSD-3-Clause",
   "Operating System :: OS Independent",
   "Programming Language :: Python",
   "Programming Language :: Python :: 3",