>> "Convert a mapping object or a sequence of two-element tuples, which
>> may either be a str or a bytes, to a “percent-encoded” string."
>
> Mappings, duple sequences, and duples cannot be str or bytes.

The only thing making sense to me here is that “which” refers to “element”.  Rephrased: Convert a mapping or sequence of two-element tuples, where each mapping key or each first element in the tuples may be str or bytes.