* set sq_repeat and sq_concat to NULL for user-defined new-style
  classes, as a way to fix a number of related problems.  See
  test_descr.notimplemented()).  One of these problems was fixed
  in r25556 and r25557 but many more existed; this is a general
  fix and thus reverts r25556-r25557.

* to avoid having PySequence_Repeat()/PySequence_Concat() failing
  on user-defined classes, they now fall back to nb_add/nb_mul if
  sq_concat/sq_repeat are not defined and the arguments appear to
  be sequences.

* added tests.

Backport candidate.