Use SPDX license identifier

For automatic dependency scanners the SPDX ID is better since it is
unambiguous. "new BSD License" could mean a lot looking at all the BSD
licenses: https://spdx.org/licenses/

setup.py

@@ -13,7 +13,7 @@
           author='Michele Simionato',
           author_email='[email protected]',
           url='https://github.com/micheles/decorator',
-          license="new BSD License",
+          license="BSD-2-Clause",
           package_dir={'': 'src'},
           py_modules=['decorator'],
           keywords="decorators generic utility",