* Fix a possible reference leak in _PyErr_Print() if exception
  is NULL.
* PyErr_BadInternalCall(): replace PyErr_Format() with _PyErr_SetString().
* Add pycore_pyerrors.h header file.
* New functions:

  * _PyErr_Clear()
  * _PyErr_Fetch()
  * _PyErr_Print()
  * _PyErr_Restore()
  * _PyErr_SetObject()
  * _PyErr_SetString()

* Add 'tstate' parameter to _PyEval_AddPendingCall().