fix: use setuptools.find_namespace_packages (#246)

Similar to googleapis/google-auth-library-python#1205 https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

Author: parthea

setup.py

@@ -82,7 +82,7 @@
 # benchmarks, etc.
 packages = [
  package
- for package in setuptools.PEP420PackageFinder.find()
+ for package in setuptools.find_namespace_packages()
  if package.startswith("bigframes") or package.startswith("third_party")
 ]