Use a valid SPDX identifier for the license

As per <https://spdx.org/licenses/>

Author: maresb

setup.py

@@ -25,7 +25,7 @@
 AUTHOR = "PyMC Developers"
 AUTHOR_EMAIL = "[email protected]"
 URL = "http://github.com/pymc-devs/pymc-extras"
-LICENSE = "Apache License, Version 2.0"
+LICENSE = "Apache-2.0"
 
 classifiers = [
     "Development Status :: 5 - Production/Stable",