Fix license specifier

Closes #2470

Author: jeromekelleher

pyproject.toml

@@ -6,7 +6,7 @@ authors = [
 description = "Simulate genealogical trees and genomic sequence data using population genetic models"
 readme = "README.md"
 requires-python = ">=3.10"
-license = {text = "GNU GPLv3+"}
+license = "GPL-3.0-or-later"
 classifiers = [
     "Programming Language :: C",
     "Programming Language :: Python",
@@ -20,7 +20,6 @@ classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Environment :: Other Environment",
     "Intended Audience :: Science/Research",
-    "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
     "Operating System :: POSIX",
     "Operating System :: MacOS :: MacOS X",
     "Operating System :: Microsoft :: Windows",