Switch to SPDX license identifier in setup.cfg

Author: cottsay

setup.cfg

@@ -13,13 +13,12 @@ classifiers =
     Development Status :: 3 - Alpha
     Environment :: Plugins
     Intended Audience :: Developers
-    License :: OSI Approved :: Apache Software License
     Operating System :: MacOS
     Operating System :: Microsoft :: Windows
     Operating System :: POSIX
     Programming Language :: Python
     Topic :: Software Development :: Build Tools
-license = Apache License, Version 2.0
+license = Apache-2.0
 description = Extension for colcon to output package information.
 long_description = file: README.rst
 keywords = colcon