- Fixed bug where some exception re-raise scenarios would attach

- Fixed bug where some exception re-raise scenarios would attach
the exception to itself as the "cause"; while the Python 3 interpreter
is OK with this, it could cause endless loops in iPython.
fixes #3625
- add tests for reraise, raise_from_cause
- raise_from_cause is the same on py2k/3k, use just one function

(cherry picked from commit d4d9a6524886eb33644e8ce42212267fa569e555)