py/objexcept: Allow clearing traceback with 'exc.__traceback__ = None'.
We allow 'exc.__traceback__ = None' assignment as a low-level optimization of pre-allocating exception instance and raising it repeatedly - this avoids memory allocation during raise. However, uPy will keep adding traceback entries to such exception instance, so before throwing it, traceback should be cleared like above. 'exc.__traceback__ = None' syntax is CPython compatible. However, unlike it, reading that attribute or setting it to any other value is not supported (and not intended to be supported, again, the only reason for adding this feature is to allow zero-memalloc exception raising).