Pivot/Transpose pandas dataframe

Question

I need to pivot a pandas dataframe

ID	A1	A2	B1	B2	C1	C2
1	100	102	99	103	100	100
2	90	92	89	93	90	90
3	120	122	119	123	120	120

into the following form

ID	label_1	label_2
1	A1	100
1	A2	102
1	B1	99
1	B2	103
1	C1	100
1	C2	100
2	A1	90
2	A2	92
2	B1	89
2	B2	93
2	C1	90
2	C2	90
3	A1	120
3	A2	122
3	B1	119
3	B2	123
3	C1	120
3	C2	120

I tried different combinations of resetting the index, transposing and pivoting without any success.