.gitlab-ci.yml: Add test environment for Python 3.8

* Bump the testsuite image versions in order to get the latest Python
  3.8 image, as we need at least Python 3.8.1. Python 3.8.0 suffers from
  an issue that causes recursion errors when using `shutil.copytree`.
  See upstream issue (https://bugs.python.org/issue38688) for more
  details on that.

* Add partial support for running tests on Python 3.8. Coverage still
  doesn't play well with our tests on 3.8.
  https://gitlab.com/BuildStream/buildstream/issues/1173 is tracking
  that.

.gitlab-ci.yml

@@ -24,12 +24,12 @@ variables:
   # Our own variables
   # Version of the docker images we should use for all the images.
   # This is taken from buildstream/buildstream-docker-images
-  DOCKER_IMAGE_VERSION: master-103717922
+  DOCKER_IMAGE_VERSION: master-105004115
   PYTEST_ADDOPTS: "--color=yes"
   INTEGRATION_CACHE: "${CI_PROJECT_DIR}/cache/integration-cache"
   PYTEST_ARGS: "--color=yes --integration -n 2"
   TEST_COMMAND: "tox -- ${PYTEST_ARGS}"
-  EXTERNAL_TESTS_COMMAND: "tox -e py35-external,py36-external,py37-external -- ${PYTEST_ARGS}"
+  EXTERNAL_TESTS_COMMAND: "tox -e py35-external,py36-external,py37-external,py38-external -- ${PYTEST_ARGS}"
   COVERAGE_PREFIX: "${CI_JOB_NAME}."
 
 
@@ -88,6 +88,21 @@ tests-centos-7.6:
   <<: *tests
   image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-centos:7.6.1810-${DOCKER_IMAGE_VERSION}
 
+tests-python-3.8-buster:
+  image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-python:3.8-buster-${DOCKER_IMAGE_VERSION}
+  <<: *tests
+  variables:
+    # Our testsuite has issues with coverage on Python 3.8 so disable coverage
+    # in the meantime. For more details, see
+    # https://gitlab.com/BuildStream/buildstream/issues/1173.
+    TEST_COMMAND: "tox -e py38-nocover -- ${PYTEST_ARGS}"
+
+    # This particular testsuite image has both Python 3.7 and Python 3.8 so we
+    # need to explicitly force the 3.8 environment.
+    # Once Python 3.8 is available in distros, we should switch to such an
+    # image, and remove the following override.
+    EXTERNAL_TESTS_COMMAND: "tox -e py38-external -- ${PYTEST_ARGS}"
+
 overnight-fedora-30-aarch64:
   image: registry.gitlab.com/buildstream/buildstream-docker-images/testsuite-fedora:aarch64-30-${DOCKER_IMAGE_VERSION}
   tags:
@@ -397,8 +412,8 @@ overnight-randomized:
   - chown -R buildstream:buildstream .
 
   # Don't run tests multiprocessed here, the randomized order doesn't like that
-  - su buildstream -c "tox -e py35-randomized,py36-randomized,py37-randomized -- --color=yes --integration"
-  - su buildstream -c "tox -e py35-randomized-external,py36-randomized-external,py37-randomized-external -- --color=yes --integration"
+  - su buildstream -c "tox -e py35-randomized,py36-randomized,py37-randomized,py38-randomized -- --color=yes --integration"
+  - su buildstream -c "tox -e py35-randomized-external,py36-randomized-external,py37-randomized-external,py38-randomized-external -- --color=yes --integration"
 
   # We need to override the exclusion from the template in order to run on schedules
   except: []