Update node.py: `!= False`

[galenseilis]

I don't think != False is necessary here.

[galenseilis]

Looks like the version of Sphinx might need to be updated.

[geraintpalmer]

Here we are explicitly checking if self.priority_preempt is False or not. Although I believe leaving out the != False works, the other values that self.priority_preempt can take are all strings, not True, and so to be explicit and to avoid any unintended behaviour, I think this should be retained.