[ty] Do not suggest argument completion when at value of keyword argument

[dylwil3]

In the following situation:

def foo(y_true,y_pred): ...

y_true = 1
y_pred = 2

foo(y_true=y<CURSOR>)

the completion suggestions began with y_true= and y_pred=. But it is never the right thing to suggest an argument completion (i.e. something=) when the cursor is at the value of a keyword argument. So this PR introduces an early exit to the logic for deciding to suggest arguments.

[dylwil3]

(I didn't find an existing issue/PR for this but it's both possible and probable that I'm bad at searching GitHub.)