Bump coverage version, fix coverage on Python 3.8
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requirements: Bump coverage version
Bump coverage version to 5.0.3. Among other things, this includes fixes that fix our issues with coverage reporting on Python 3.8.
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.gitlab-ci.yml: Enable coverage reporting for Python 3.8
Since bumping coverage verion, our coverage reporting works fine with Python 3.8.
Fixes #1173.