chore: fix dependabot not checking poetry dependencies

[Panaetius]

I think open-pull-requests-limit: 0 used to mean "unlimited" but now it means "don't check"

[coveralls]

Pull Request Test Coverage Report for Build 4783634085

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.005%) to 85.294%

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Files with Coverage Reduction	New Missed Lines	%
renku/ui/cli/init.py	1	96.9%
renku/ui/cli/service.py	1	68.59%
renku/command/rollback.py	2	78.7%

Totals
Change from base Build 4766838306:	0.005%
Covered Lines:	25480
Relevant Lines:	29873

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💛 - Coveralls