[Python] Reshape wide to long in pandas

Let's assume that I have the following dataframe in pandas:

AA BB CC
date
05/03 1 2 3
06/03 4 5 6
07/03 7 8 9
08/03 5 7 1


and I want to transform it to the following:

AA 05/03 1
AA 06/03 4
AA 07/03 7
AA 08/03 5
BB 05/03 2
BB 06/03 5
BB 07/03 8
BB 08/03 7
CC 05/03 3
CC 06/03 6
CC 07/03 9
CC 08/03 1


How can I do it?

The reason of the transformation from wide to long is that, in the next stage, I would like to merge this dataframe with another one, based on dates and the initial column names (AA, BB, CC).

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