Defer formatting task name

The default task name is "Task-<counter>" (if no name is passed in during Task creation).
This is initialized in `Task.__init__` (C impl) using string formatting, which can be quite slow.
Actually using the task name in real world code is not very common, so this is wasted init.

Let's defer this string formatting to the first time the name is read (in `get_name` impl),
so we don't need to pay the string formatting cost if the task name is never read.

Modules/_asynciomodule.c

@@ -129,6 +129,7 @@ typedef struct {
     int task_must_cancel;
     int task_log_destroy_pending;
     int task_num_cancels_requested;
+    uint64_t task_name_counter;
 } TaskObj;
 
 typedef struct {
@@ -2065,12 +2066,16 @@ _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop,
     self->task_must_cancel = 0;
     self->task_log_destroy_pending = 1;
     self->task_num_cancels_requested = 0;
+    self->task_name_counter = 0;
     Py_INCREF(coro);
     Py_XSETREF(self->task_coro, coro);
 
     if (name == Py_None) {
-        name = PyUnicode_FromFormat("Task-%" PRIu64,
-                                    ++state->task_name_counter);
+        // optimization: defer task name formatting
+        // set task_name to None to indicate deferred formatting, and
+        // store the task name counter for formatting in get_name impl
+        Py_INCREF(name);
+        self->task_name_counter = ++state->task_name_counter;
     } else if (!PyUnicode_CheckExact(name)) {
         name = PyObject_Str(name);
     } else {
@@ -2448,6 +2453,13 @@ static PyObject *
 _asyncio_Task_get_name_impl(TaskObj *self)
 /*[clinic end generated code: output=0ecf1570c3b37a8f input=a4a6595d12f4f0f8]*/
 {
+    if (Py_IsNone(self->task_name)) {
+        assert(self->task_name_counter > 0);
+        PyObject *name = PyUnicode_FromFormat(
+            "Task-%" PRIu64, self->task_name_counter);
+        Py_XSETREF(self->task_name, name);
+        return Py_NewRef(self->task_name);
+    }
     if (self->task_name) {
         return Py_NewRef(self->task_name);
     }