`UniProt.get_sequence` should not raise an exception when >1 IDs for canonical sequence are found and isoform is provided

[mtiberti]

Currently, we raise a RuntimeError when we find entries with >1 IDs for the canonical sequence, whether the isoform argument was provided or not

However, legit cases in which we find >1 IDs for the canonical sequence do exist (e.g. P61812) and we should be able to handle them when isoform is selected (so that the user can decide which ID they would like to refer to)