Bring Python interop tests to the beta API

Also uses the .proto files for the service/stub definitions.
diff --git a/src/python/grpcio_test/setup.py b/src/python/grpcio_test/setup.py
index 0f43b4a..e9ee45a 100644
--- a/src/python/grpcio_test/setup.py
+++ b/src/python/grpcio_test/setup.py
@@ -77,7 +77,9 @@
 )
 
 _COMMAND_CLASS = {
-    'test': commands.RunTests
+    'test': commands.RunTests,
+    'build_proto_modules': commands.BuildProtoModules,
+    'build_py': commands.BuildPy,
 }
 
 setuptools.setup(