THRIFT-5024: Replace removed @gen.engine with @gen.coroutine
Client: py
Patch: Asjad Syed

This closes #3111
diff --git a/compiler/cpp/src/thrift/generate/t_py_generator.cc b/compiler/cpp/src/thrift/generate/t_py_generator.cc
index 893d635..c34bb43 100644
--- a/compiler/cpp/src/thrift/generate/t_py_generator.cc
+++ b/compiler/cpp/src/thrift/generate/t_py_generator.cc
@@ -1460,7 +1460,7 @@
 
   if (gen_tornado_ && extends.empty()) {
     f_service_ << '\n' <<
-      indent() << "@gen.engine" << '\n' <<
+      indent() << "@gen.coroutine" << '\n' <<
       indent() << "def _start_receiving(self):" << '\n';
     indent_up();
     indent(f_service_) << "while True:" << '\n';