Use limited Python API where possible (#568)

Use limited Python API to help address rapidsai/build-planning#42.

Authors:
  - Peter Andreas Entschev (https://github.com/pentschev)
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)
  - https://github.com/jakirkham

URL: #568

Author: pentschev

cpp/python/src/future.cpp

@@ -95,10 +95,7 @@ PyObject* create_python_future()
   future_object = get_asyncio_future_object();
   if (future_object == NULL) { goto finish; }
   if (!PyCallable_Check(future_object)) {
-    PyErr_Format(PyExc_RuntimeError,
-                 "%s.%s is not callable.",
-                 PyUnicode_1BYTE_DATA(asyncio_str),
-                 PyUnicode_1BYTE_DATA(future_str));
+    PyErr_Format(PyExc_RuntimeError, "%U.%U is not callable.", asyncio_str, future_str);
     goto finish;
   }
 
@@ -251,10 +248,7 @@ PyObject* future_set_result_with_event_loop(PyObject* event_loop, PyObject* futu
     goto finish;
   }
   if (!PyCallable_Check(set_result_callable)) {
-    PyErr_Format(PyExc_RuntimeError,
-                 "%s.%s is not callable.",
-                 PyUnicode_1BYTE_DATA(future),
-                 PyUnicode_1BYTE_DATA(set_result_str));
+    PyErr_Format(PyExc_RuntimeError, "%R.%U is not callable.", future, set_result_str);
     goto finish;
   }
 
@@ -306,10 +300,7 @@ PyObject* future_set_exception_with_event_loop(PyObject* event_loop,
     goto finish;
   }
   if (!PyCallable_Check(set_exception_callable)) {
-    PyErr_Format(PyExc_RuntimeError,
-                 "%s.%s is not callable.",
-                 PyUnicode_1BYTE_DATA(future),
-                 PyUnicode_1BYTE_DATA(set_exception_str));
+    PyErr_Format(PyExc_RuntimeError, "%R.%U is not callable.", future, set_exception_str);
     goto finish;
   }