Like most people here already said, you shouldn't handle your BP while he's shedding unless you absolutely have to. In this stage most BPs are irritable and don't act like they usually do. I say most, because my Albino doesn't care about being handled during shed but my normal definitely had mood swings. So it really depends on the snake. The hissing is your BP being "thoughtful" and letting you know he's not happy right now.
On the handling part, you don't need to handle BPs much in order for them to be used to it. My advice is that you should actually limit yourself to handling your BP to a couple of times a week because excessive handling might be stressful to the little guy.
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1.0 Abino, 0.1 Normal, 0.1 Pastave Het Pied