logilab-astng #47957 write tests and inference for Set node [validation pending]

add a Set node like a Tuple or List node. Write tests and inference method

prioritynormal
typetask
done in<not specified>
load0.500
load left0.000
closed by#41db38a1c28e Set literals should be treated as inference leaves. Closes #47957
patchSet literals should be treated as inference leaves. [applied]