The built-in function sum takes an optional argument "start" to specify what value to start adding from (defaults to 0). This argument should be a keyword argument in order to match the other built-in functions such as:

    enumerate(range(10), start=5)

This patch allows users to write:

    sum(range(10), start=5)

which previously raised "TypeError: sum() takes no keyword arguments". Since the only change is making an optional positional argument into a keyword argument, this has no effect on any existing code using the current convention of:

    sum(range(10), 5)