Make `.get_part` an class method

[curlup]

It basically doesn't need access to self, only to class itself, right?

python-json-pointer/jsonpointer.py

Line 231 in 277e106

def get_part(self, doc, part):

[stefankoegl]

Hi! That's right. Could you submit a corresponding pull request?

[stefankoegl]

I have just released v2.2 with the change from #46