Add compat.reraise to handle python 3 syntax breakage

Add compat.reraise to handle python 3 syntax breakage

Add a exception reraise function to compat that will allow us to
rewrite invocations like this:

    except Exception as e:
        raise ClientError, e, sys.exc_info()[2]

as this:

    except Exception as e:
        reraise(ClientError(e))

since python 3 now provides a with_traceback() method that we can (and
must) use instead.

Put the python 2 specific formulation (shown above) in a separate
py2raise module that we can conditionally import because python 3
decided to make the python 2 code produce a syntax error.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>