re-enable coverage tests

We somehow need to tell coveragepy that we're running in parallel even
though it's unclear to me that we do:

https://github.com/nedbat/coveragepy/issues/702#issuecomment-432016361

Just doing what I'm told and tests pass locally.

This reverts "disable cov which crashes on sqlite3.IntegrityError",
commit 69770cbf.
parent 5598d3f5
Pipeline #33938787 (#155) passed with stage
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