py3: don't use traceback.print_exc() in commandserver.py
It doesn't support a bytes stream on Python 3. This makes a traceback being
sent by one frame, but that shouldn't matter.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL