python3-testtools: Avoid traceback2 module requirement
traceback2 adds traceback for python2. Rather than depend on traceback2, we're
python3 only so just use traceback.
This caused breakage in oe-selftest -j which uses testtools on the autobuilder
using buildtools-tarball.
[YOCTO #13652]
(From OE-Core rev:
ee80a06c107375e3cf0d246ea17c09dda4536dab)
(From OE-Core rev:
ee82e3c24fe5727ce81e972cadedca431d6086c5)
(From OE-Core rev:
be4470c9590183b388d9ff176331d0c50984dec8)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
