Fix deprecated use of namespace_packages in python/dist/setup.py (#18235)

Uses implicit namespaces instead of explicit, as per https://peps.python.org/pep-0420/

Fixes a warning otherwise output during build.

Closes #18235

COPYBARA_INTEGRATE_REVIEW=#18235 from tempoz:tempoz-fix-deprecated-use-of-namespace-packages 85067b6
PiperOrigin-RevId: 700426731

Author: tempoz

python/dist/setup.py

@@ -12,7 +12,7 @@
 import os
 import sys
 
-from setuptools import setup, Extension, find_packages
+from setuptools import setup, Extension, find_namespace_packages
 
 
 def GetVersion():
@@ -59,8 +59,7 @@ def GetVersion():
         'Programming Language :: Python :: 3.12',
         'Programming Language :: Python :: 3.13',
     ],
-    namespace_packages=['google'],
-    packages=find_packages(),
+    packages=find_namespace_packages(include=['google*']),
     install_requires=[],
     ext_modules=[
         Extension(