[python-hl] Fix python highlighting of some operators
[python-hl] Fix python highlighting of some operators

Adds operator highlighting for:
    @   This is matrix multiplication, but care needs to be taken when
        it is part of a decorator.

    /
    *
    **  These when used in function signatures (* indicates varargs,
        ** keyword args and / stop of non-keyword args). They're not strictly
        operators but there is no easy way to tell the usage apart from operator
        usage so operator highlighting is used here as well.

Removes operator highlighting for:
    ->  This is used in type signatures, but just like : is not an operator
    <>  This is not a python operator

danr/kakoune,danr/kakoune,danr/kakoune,danr/kakoune