This PR fixes #804

Here is original zoom wheel handler code:

def wheel(self, event):
    self.zoomCanvas(event.x, event.y, pow(ZOOM,(event.delta//120)))

event.delta equals -1 (scroll up) or 1 (scroll down) on my setup (MacOS 12.2.1, Logitech MX 3 mouse, medium scrolling speed in system settings, "natural" scroll direction). In Python 2 rounding goes away from zero, so event.delta//120 gives either 1 or -1, which is OK. But running in Python 3.9, event.delta//120 results -1 or 0. That why only one zooming direction works.

I propose a fix that seems compatible both with Python 2 and 3 and should not break or change scrolling behavior.