python: change types for RBufferOut/FBuffer with Python 3 (RHBZ#1661871)

So far RBufferOut return values, and FBuffer struct fields are 'str' on
all the versions of Python.  Python 3 distinguishes between 'str'
(unicode strings), and 'bytes', with 'str' no more able to hold
arbitrary data.

For this reason, switch the return value of RBufferOut functions, and
FBuffer struct fields to bytes on Python 3: while this is a potentially
incompatibile change, this is the only way to handle safely sequences
of arbitrary bytes.

generator/python.ml

@@ -195,8 +195,13 @@ and generate_python_structs () =
             pr "    goto err;\n";
             pr "  PyDict_SetItemString (dict, \"%s\", value);\n" name;
         | name, FBuffer ->
+            pr "#if PY_MAJOR_VERSION >= 3\n";
+            pr "  value = PyBytes_FromStringAndSize (%s->%s, %s->%s_len);\n"
+              typ name typ name;
+            pr "#else\n";
             pr "  value = guestfs_int_py_fromstringsize (%s->%s, %s->%s_len);\n"
               typ name typ name;
+            pr "#endif\n";
             pr "  if (value == NULL)\n";
             pr "    goto err;\n";
             pr "  PyDict_SetItemString (dict, \"%s\", value);\n" name;
@@ -511,7 +516,11 @@ and generate_python_actions actions () =
            pr "  guestfs_int_free_string_list (r);\n";
            pr "  if (py_r == NULL) goto out;\n";
        | RBufferOut _ ->
+           pr "#if PY_MAJOR_VERSION >= 3\n";
+           pr "  py_r = PyBytes_FromStringAndSize (r, size);\n";
+           pr "#else\n";
            pr "  py_r = guestfs_int_py_fromstringsize (r, size);\n";
+           pr "#endif\n";
            pr "  free (r);\n";
            pr "  if (py_r == NULL) goto out;\n";
       );