Things like Decimal('0.200000000000000011102230246251565404236316680908203125') are ugly IMO. But more importantly, the test is confusing because Decimal is normally used with strings in input. If you use floats, Python converts them in strings with the default precision of the Decimal type. So I read the test and think: why it is important to pass as input  Decimal('0.200000000000000011102230246251565404236316680908203125') and not Decimal('0.2')?
The comment confused me too. I would rephrase the comment as follows:

``This test checks that the function post_processing.weighted_quantile_curve works even when the weights are passed as Decimals.``

I assume that the weights are passed as Decimals to make sure that the sum is *exactly* 1. However, since at the end they are converted into floats anyway I am not sure if this complication is buying us anything.