allow mixed str and dict in lint config

[mashehu]

we should probably rewrite this into a TypedDict or similar in the future.

[Joon-Klaps]

Could you write a quick test for this as well?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.87%. Comparing base (dabf1ef) to head (eed0598). Report is 52 commits behind head on dev.

Files with missing lines	Patch %	Lines
nf_core/pipelines/lint/nfcore_yml.py	76.92%	3 Missing :warning:
nf_core/pipelines/sync.py	78.57%	3 Missing :warning:

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[Joon-Klaps]

Thanks @mashehu ❤️