change: stop CI unit test code checks from running in parallel (#1066)

There seems to be a ~5% failure rate when the code checks are run in
parallel. This change adds a minute to the unit tests running
in the CI pipeline, but avoids the confusing failure.

buildspec-unittests.yml

@@ -7,7 +7,7 @@ phases:
       - TOX_PARALLEL_NO_SPINNER=1
       - PY_COLORS=0
       - start_time=`date +%s`
-      - tox -e flake8,pylint,twine,black-check --parallel all
+      - tox -e flake8,pylint,twine,black-check
       - ./ci-scripts/displaytime.sh 'flake8,pylint,twine,black-check' $start_time
 
       - start_time=`date +%s`