Unpin coverage<6.3

[akihironitta]

What does this PR do?

Follows up #11635 as nedbat/coveragepy#1310 (comment) is resolved.

Does your PR introduce any breaking changes? If yes, please list them.

None

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Did you have fun?

Make sure you had fun coding 🙃

cc @carmocca @akihironitta @Borda