Help to melt data using pandas

Python Help
1

I’m using pandas to processing my raw data.
I have raw data like this:

pic1
pic11016×118 5.54 KB

I want to convert some columns to row using pd.melt like this:
pic2
pic2544×533 10.3 KB

But, I don’t know how to move USL & LSL transpose to 2 columns.
Can anyone help me.
Thanks so much!!!
My file:

https://docs.google.com/spreadsheets/d/1R-_4YZSXFa00ZewADVpSlmK1nSlETVDx/edit?usp=sharing&ouid=100090612001289608062&rtpof=true&sd=true