use 'callable' to check if object is callable (B004)

[danieleades]

from the bugbear docs-

B004: Using hasattr(x, 'call') to test if x is callable is unreliable. If x implements custom getattr or its call is itself not callable, you might get misleading results. Use callable(x) for consistent results.

also, it looks better.

[tk0miya]

Cool!