refactor PyPiRequirement to use serde_untagged

[baszalmstra]

I stumbled upon serde_untagged and I think it would be very useful to remove some of the complexity of the Deserialize impl for PyPiRequirement. See:

pixi/src/project/python.rs

Line 134 in dc5bab5

impl<'de> Deserialize<'de> for PyPiRequirement {

Might be a nice "first issue".

[marcelotrevisani]

I am not sure if this is what you meant, but I am happy to give it a go
#622