don't narrow locals if there is a nonlocal reference to it

[KotlinIsland]

playground

def f():
    def m():
        nonlocal a
        a += 1
    a = 1
    m()
    if a == 2: # error 1 != 2
        print("hmm")     

here, the nonlocal a is a direct indicator that a will be mutated and should therefore not be narrowed:

def f():
    def m():
        nonlocal a
        a += 1
    a = 1
    reveal_type(a)  # int
    m()
    if a == 2: # no error
        print("hmm")     

in a more complex analysis, we could only apply these widenings after the appropriate function (m) is invoked, this sounds plausible to do, but a lot more work