devshell.bbclass: Exceptions displayed within devpyshell

If you type something into devpyshell, an exception should be triggered,
but that exception isn't printed out because it is not being flushed out
of the buffer. This patch fixes that issue.

[YOCTO #12156]

Signed-off-by: Dorinda Bassey <dorindabassey@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 9f95e82a)
Signed-off-by: Steve Sakoman <steve@sakoman.com>