perf script python: Use PyBytes for attr in trace-event-python

With Python3.  PyUnicode_FromStringAndSize is unsafe to call on attr and will
return NULL.  Use _PyBytes_FromStringAndSize (as with raw_buf).

Below is the observed behavior without the fix.  Note it is first necessary
to apply the prior fix (Add trace_context extension module to sys,modules):

  # ldd /usr/bin/perf | grep -i python
          libpython3.6m.so.1.0 => /usr/lib64/libpython3.6m.so.1.0 (0x00007f8e1dfb2000)

  # perf record -e raw_syscalls:sys_enter /bin/false
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.018 MB perf.data (21 samples) ]

  # perf script -g python | cat
  generated Python script: perf-script.py

  # perf script -s ./perf-script.py
  in trace_begin
  Segmentation fault (core dumped)
Signed-off-by: Tony Jones <tonyj@suse.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jaroslav Škarvada <jskarvad@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Fixes: 66dfdff0 ("perf tools: Add Python 3 support")
Link: http://lkml.kernel.org/r/20190124005229.16146-3-tonyj@suse.deSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/scripting-engines/trace-event-python.c

@@ -733,8 +733,7 @@ static PyObject *get_perf_sample_dict(struct perf_sample *sample,
 		Py_FatalError("couldn't create Python dictionary");
 
 	pydict_set_item_string_decref(dict, "ev_name", _PyUnicode_FromString(perf_evsel__name(evsel)));
-	pydict_set_item_string_decref(dict, "attr", _PyUnicode_FromStringAndSize(
-			(const char *)&evsel->attr, sizeof(evsel->attr)));
+	pydict_set_item_string_decref(dict, "attr", _PyBytes_FromStringAndSize((const char *)&evsel->attr, sizeof(evsel->attr)));
 
 	pydict_set_item_string_decref(dict_sample, "pid",
 			_PyLong_FromLong(sample->pid));