Make license info compatible with PEP 639

https://peps.python.org/pep-0639/

Fixes issues like:
SyntaxWarning: invalid escape sequence '\['
        verb_loop = re.compile("^(    .*)@\[[a-z]*\]", re.MULTILINE)
      /root/.cache/uv/builds-v0/.tmp70O2Lb/lib/python3.13/site-packages/setupto
ols/config/_apply_pyprojecttoml.py:82:
      SetuptoolsDeprecationWarning: `project.license` as a TOML table is
      deprecated

(that's for setuptools, but meson-python should throw a similar error at some point)

Author: tobiasdiez

pyproject.toml

@@ -14,7 +14,8 @@ maintainers = [
 dependencies = ["cysignals>=1.11.3"]
 requires-python = ">=3.9"
 readme = "README.rst"
-license = { text = "GNU General Public License, version 2 or later" }
+license = "GPL-2.0-or-later"
+license-files = ["LICENSE"]
 keywords = ["PARI/GP number theory"]
 dynamic = ["version"]