perf python: Stop using deprecated PyUnicode_AsString()

As noticed while building for Fedora 43:

GEN /tmp/build/perf/python/perf.cpython-314-x86_64-linux-gnu.so
/git/perf-6.16.0-rc3/tools/perf/util/python.c: In function ‘get_tracepoint_field’:
/git/perf-6.16.0-rc3/tools/perf/util/python.c:340:9: error: ‘_PyUnicode_AsString’ is deprecated [-Werror=deprecated-declarations]
340 | const char *str = _PyUnicode_AsString(PyObject_Str(attr_name));
| ^~~~~
In file included from /usr/include/python3.14/unicodeobject.h:1022,
from /usr/include/python3.14/Python.h:89,
from /git/perf-6.16.0-rc3/tools/perf/util/python.c:2:
/usr/include/python3.14/cpython/unicodeobject.h:648:1: note: declared here
648 | _PyUnicode_AsString(PyObject *unicode)
| ^~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
error: command '/usr/bin/gcc' failed with exit code 1

Use PyUnicode_AsUTF8() instead and also check if PyObject_Str() fails
before doing so.

Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>
Link: https://lore.kernel.org/r/aIofXNK8QLtLIaI3@x1
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/util/python.c

···
 static PyObject*
 get_tracepoint_field(struct pyrf_event *pevent, PyObject *attr_name)
 {
-	const char *str = _PyUnicode_AsString(PyObject_Str(attr_name));
 	struct evsel *evsel = pevent->evsel;
 	struct tep_event *tp_format = evsel__tp_format(evsel);
 	struct tep_format_field *field;
···
 	if (IS_ERR_OR_NULL(tp_format))
 		return NULL;
 
+	PyObject *obj = PyObject_Str(attr_name);
+	if (obj == NULL)
+		return NULL;
+
+	const char *str = PyUnicode_AsUTF8(obj);
+	if (str == NULL) {
+		Py_DECREF(obj);
+		return NULL;
+	}
+
 	field = tep_find_any_field(tp_format, str);
+	Py_DECREF(obj);
 	return field ? tracepoint_field(pevent, field) : NULL;
 }
 #endif /* HAVE_LIBTRACEEVENT */