bpo-32591: _PyErr_WarnUnawaitedCoroutine() sets source (GH-19247)

The _PyErr_WarnUnawaitedCoroutine() fallback now also sets the
coroutine object as the source of the warning, as done by the Python
implementation warnings._warn_unawaited_coroutine().

Moreover, don't truncate the coroutine name: Python supports
arbitrary string length to format the message.

Python/_warnings.c

@@ -1129,6 +1129,23 @@ PyErr_WarnFormat(PyObject *category, Py_ssize_t stack_level,
     return res;
 }
 
+static int
+_PyErr_WarnFormat(PyObject *source, PyObject *category, Py_ssize_t stack_level,
+                  const char *format, ...)
+{
+    int res;
+    va_list vargs;
+
+#ifdef HAVE_STDARG_PROTOTYPES
+    va_start(vargs, format);
+#else
+    va_start(vargs);
+#endif
+    res = _PyErr_WarnFormatV(source, category, stack_level, format, vargs);
+    va_end(vargs);
+    return res;
+}
+
 int
 PyErr_ResourceWarning(PyObject *source, Py_ssize_t stack_level,
                       const char *format, ...)
@@ -1297,9 +1314,9 @@ _PyErr_WarnUnawaitedCoroutine(PyObject *coro)
         PyErr_WriteUnraisable(coro);
     }
     if (!warned) {
-        if (PyErr_WarnFormat(PyExc_RuntimeWarning, 1,
-                             "coroutine '%.50S' was never awaited",
-                             ((PyCoroObject *)coro)->cr_qualname) < 0)
+        if (_PyErr_WarnFormat(coro, PyExc_RuntimeWarning, 1,
+                              "coroutine '%S' was never awaited",
+                              ((PyCoroObject *)coro)->cr_qualname) < 0)
         {
             PyErr_WriteUnraisable(coro);
         }