use SPDX license identifier in setup.py (#503)

Author: oliver

setup.py

@@ -28,7 +28,7 @@ def find_version():
     author='Cloudpipe',
     author_email='[email protected]',
     url='https://github.com/cloudpipe/cloudpickle',
-    license='BSD 3-Clause License',
+    license='BSD-3-Clause',
     packages=['cloudpickle'],
     long_description=open('README.md').read(),
     long_description_content_type="text/markdown",