Add built in flatmap function to functools

[jeanas (Jean Abou Samra)]

Guarantees on sum() for non-numeric types

The documentation of sum() at Built-in Functions — Python 3.10.8 documentation reads: “Sums start and the items of an iterable from left to right and returns the total. The iterable’s items are normally numbers, and the start value is not allowed to be a string.” However, its docstring has a much stronger wording on summing values of non-numeric types. pydoc sum gives “This function is intended specifically for use with numeric values and may reject non-numeric types.” (and doesn’t mention t…