Switch to SPDX license identifier in setup.cfg

The classifier is now deprecated in modern setuptools in favor of using
SPDX identifiers as license values.

Author: cottsay

setup.cfg

@@ -11,12 +11,11 @@ classifiers =
   Development Status :: 3 - Alpha
   Environment :: Plugins
   Intended Audience :: Developers
-  License :: OSI Approved :: Apache Software License
   Operating System :: MacOS
   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 serve as a template for new extensions.
 long_description = file: README.rst
 keywords = colcon