[Python] Pandas DataFrame stack multiple column values into single column

Assuming the following DataFrame:

key.0 key.1 key.2 topic
1 abc def ghi 8
2 xab xcd xef 9


How can I combine the values of all the key.* columns into a single column 'key', that's associated with the topic value corresponding to the key.* columns? This is the result I want:

topic key
1 8 abc
2 8 def
3 8 ghi
4 9 xab
5 9 xcd
6 9 xef


Note that the number of key.N columns is variable on some external N.

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