Fixes old call to sort() method

[orestesmas]

SPR 'master' fails to load raw ngspice data on python 3.6

>>> from spr import load_raw
>>> data=load_raw('SR-UWB.dat')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/....(entire path isn't relevant).../site-packages/spr/__init__.py", line 65, in load_raw
    ret["vars"].sort(cmp=lambda x, y: int(x["idx"])-int(y["idx"]))
TypeError: 'cmp' is an invalid keyword argument for this function

As stated somewhere on internet:

Technical Detail: If you’re transitioning from Python 2 and are familiar with its function of the same name, you should be aware of a >couple important changes in Python 3:

Python 3’s sorted() does not have a cmp parameter. Instead, only key is used to introduce custom sorting logic.
key and reverse must be passed as keyword arguments, unlike in Python 2, where they could be passed as positional arguments.

If you need to convert a Python 2 cmp function to a key function, then check out functools.cmp_to_key(). This tutorial will not cover any examples using Python 2.