Serialising traceback objects

[cjdoris]

Python traceback objects are not picklable, and hence PyException cannot always be serialised.

As a fix, serialise the b field of a PyException as None. Or possibly always serialize traceback to None? Or to Unpicklable()?

Optional extra: if we detect that tblib is installed, allow pickling of traceback.

Document this behaviour in the compat section.

[cjdoris]

And what about other things that can't be pickled?

[cjdoris]

In fact Python Exceptions are already picklable - the __traceback__ field is skipped. So we get all this behaviour for free if we just change PyException to be a wrapper around a single Python Exception object.

[cjdoris]

Or perhaps leave the fields separate as-is, but treat PyException as a wrapper type around the v property. This lets us avoid normalising. When normalising, set v.__traceback__ to t. When serialising, just serialise v. When deserialising, set t=pytype(v) and b=v.__traceback__ and isnormalised=true.