fix: remove pluggy constraint

[ghassanmas]

Pytest have released 6.2.5 two days ago: release note : https://docs.pytest.org/en/6.2.x/changelog.html#pytest-6-2-5-2021-08-29

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[ghassanmas]

Here is where the make upgrate is being breaking at this branch openedx/xblock-in-video-quiz#24,
Here is the expected output error

[nedbat]

I ran a "make upgrade", and it succeeded without conflicts.

[ghassanmas]

@nedbat all good, thank you for the quick response 🙏

[natabene]

@ghassanmas Thank you for your contribution!

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