build(pyproject): adopt PEP 639 license metadata

Use SPDX license expressions and add license-files in pyproject.toml.
Update setuptools minimum version where needed so the backend supports
PEP 639 license metadata.

Authored by OpenClaw (model: gpt-5.4)

Author: njzjz-bot

pyproject.toml

@@ -12,7 +12,8 @@ description = "A Python template."  # Please revise this line.
 authors = [
     { name = "Jinzhe Zeng", email = "jinzhe.zeng@ustc.edu.cn"},
 ]
-license = { file = "LICENSE" }
+license = "LGPL-3.0-or-later"
+license-files = ["LICENSE"]
 classifiers = [
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",