MNT: replace isinstance if-else blocks with single-dispatch generics

[tangkong]

https://docs.python.org/3/library/functools.html#functools.singledispatch
It could be used to remove the if/isinstance block boilerplate here. (Ken) had used it (a total of once) in the pvaccess stuff in caproto and thought it was neat.

Originally posted by @klauer in #193 (comment)

This could be the right programming pattern here, but to the best of my understanding it's not adding much in terms of functionality. Maybe this helps readability.

This would have to be applied to all the other places we isinstance-spam.