fix: type hint

[jlsneto]

the document and collection functions can receive str or tuple of this
this goes up a false positive warning in the development

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Fixes #<issue_number_goes_here> 🦕

[craiglabenz]

LGTM

[tseaver]

@jlsneto Thanks for the patch! Actually, the correct type hint here per PEP 484 is just str.

[tseaver]

Superseded by #300.