Convert windows dll filename to bytes

src/python/grpcio/grpc/_cython/cygrpc.pyx

@@ -53,6 +53,8 @@ def _initialize():
   if 'win32' in sys.platform:
     filename = pkg_resources.resource_filename(
         'grpc._cython', '_windows/grpc_c.64.python')
+    if not isinstance(filename, bytes):
+      filename = filename.encode()
     if not pygrpc_load_core(filename):
       raise ImportError('failed to load core gRPC library')
   if not pygrpc_initialize_core():