Add test environment for Python 3.8

Chandan Singhrequested to merge
chandan/python3.8 into master

This partially fixes #1173 but running tests with coverage is still broken.

  • _scheduler/scheduler.py: Enforce SafeChildWatcher

    In Python 3.8, ThreadedChildWatcher is the default watcher that causes issues with our scheduler. Enforce use of SafeChildWatcher.

  • tox.ini: Add python3.8 to default environment list

  • .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. #1173 is tracking that.

Edited by Chandan Singh

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