license: switch from GPLv3 to GPLv3+ (#37)

Author: yunjunz

README.md

@@ -1,7 +1,7 @@
 [![Language](https://img.shields.io/badge/python-3.6%2B-blue.svg)](https://www.python.org/)
 [![CircleCI](https://img.shields.io/circleci/build/github/insarlab/PySolid.svg?logo=circleci&label=test)](https://circleci.com/gh/insarlab/PySolid)
 [![Version](https://img.shields.io/github/v/release/insarlab/PySolid?color=green)](https://github.com/insarlab/PySolid/releases)
-[![License](https://img.shields.io/badge/license-GPLv3-yellow.svg)](https://github.com/insarlab/PySolid/blob/main/LICENSE)
+[![License](https://img.shields.io/badge/license-GPLv3+-yellow.svg)](https://github.com/insarlab/PySolid/blob/main/LICENSE)
 [![Citation](https://img.shields.io/badge/doi-10.1109%2FTGRS.2022.3168509-blue)](https://doi.org/10.1109/TGRS.2022.3168509)
 
 ## PySolid

setup.py

@@ -23,13 +23,14 @@
     download_url=("https://github.com/insarlab/PySolid/archive/v{}.tar.gz".format(version)),
     author="Zhang Yunjun, Dennis Milbert",
     author_email="yunjunzgeo@gmail.com",
-    license_files=('LICENSE',),
+    license="GPL-3.0-or-later",
+    license_files=("LICENSE",),
 
     classifiers=[
         "Development Status :: 4 - Beta",
         "Intended Audience :: Science/Research",
         "Topic :: Scientific/Engineering",
-        "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
+        "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
         "Operating System :: OS Independent",
         "Programming Language :: Python :: 3",
     ],