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/protobuf_distutils/setup.py b/python/protobuf_distutils/setup.py
index 1092930..9a19032 100644
--- a/python/protobuf_distutils/setup.py
+++ b/python/protobuf_distutils/setup.py
@@ -46,7 +46,7 @@
     packages=find_packages(),
     maintainer='protobuf@googlegroups.com',
     maintainer_email='protobuf@googlegroups.com',
-    license='3-Clause BSD License',
+    license='BSD-3-Clause',
     classifiers=[
         "Framework :: Setuptools Plugin",
         "Operating System :: OS Independent",