Reverse Polish Notation calculator in Python
In Python, list is designed to grow efficiently from the end. Implementing a stack using append and pop at the end is therefore a better approach.
I would expect is_number to return False instead of the implicit None which usually stands for a missing value.
An explicit math. in front of functions makes clear where they come from. If you want to support more functions you have other things to worry about. For example, the forced conversion from degrees to radians only makes sense with certain functions.
You need to store explicitly the number of arguments each operator expects. Your current solution is to infer that from the length of the operator's name. That is not an extensible approach.