ImageMath: Inline isinstance check

[hugovk]

_isconstant() is short and only called once, we might as well inline it, it's also quicker:

❯ python -m timeit -s "def _isconstant(v): return isinstance(v, (int, float))" "_isconstant(12.34)"
5000000 loops, best of 5: 54.1 nsec per loop

❯ python -m timeit "isinstance(12.34, (int, float))"
10000000 loops, best of 5: 38.3 nsec per loop

History:

The current one is from 2019:

def _isconstant(v):
    return isinstance(v, (int, float))

The 2013, 2to3 version was:

def _isconstant(v):
    return isinstance(v, int) or isinstance(v, float)

The original 2006, PIL 1.1.6 version was:

def _isconstant(v):
    return isinstance(v, type(0)) or isinstance(v, type(0.0))