Clean up Decimal to utf8 convertion and switch to using PyObject_Form…

…at() to suppress scientific notation

pandas/_libs/src/vendored/ujson/python/objToJSON.c

@@ -376,25 +376,18 @@ static char *PyTimeToJSON(JSOBJ _obj, JSONTypeContext *tc, size_t *outLen) {
 static char *PyDecimalToUTF8Callback(JSOBJ _obj, JSONTypeContext *tc,
                                      size_t *len) {
   PyObject *obj = (PyObject *)_obj;
-  PyObject *str = PyObject_Str(obj);
+  PyObject *format_spec = PyUnicode_FromStringAndSize("f", 1);
+  PyObject *str = PyObject_Format(obj, format_spec);
+  Py_DECREF(format_spec);
+
   if (str == NULL) {
     *len = 0;
-    if (!PyErr_Occurred()) {
-      PyErr_SetString(PyExc_ValueError, "Failed to convert decimal");
-    }
     ((JSONObjectEncoder *)tc->encoder)->errorMsg = "";
     return NULL;
   }
-  if (PyUnicode_Check(str)) {
-    PyObject *tmp = str;
-    str = PyUnicode_AsUTF8String(str);
-    Py_DECREF(tmp);
-  }
 
   GET_TC(tc)->newObj = str;
-
-  *len = PyBytes_GET_SIZE(str);
-  char *outValue = PyBytes_AS_STRING(str);
+  char *outValue = (char *)PyUnicode_AsUTF8AndSize(str, (Py_ssize_t *)len);
   return outValue;
 }