Use proper SPDX license identifier (#9441)

The previously used term "3-Clause BSD License" is not properly
standarized. A common standard is SPDX, therefore "3-Clause BSD License"
is substituted with "BSD-3-Clause" which is a SPDX identifier.

`grep -rl "3-Clause BSD License" | xargs -n1 sed -i "s/3-Clause BSD
License/BSD-3-Clause/g"`
diff --git a/python/setup.py b/python/setup.py
index 77d9cd1..87dff52 100755
--- a/python/setup.py
+++ b/python/setup.py
@@ -283,7 +283,7 @@
       url='https://developers.google.com/protocol-buffers/',
       maintainer='protobuf@googlegroups.com',
       maintainer_email='protobuf@googlegroups.com',
-      license='3-Clause BSD License',
+      license='BSD-3-Clause',
       classifiers=[
           "Programming Language :: Python",
           "Programming Language :: Python :: 3",