Path.contains_points() returns a uint8 array instead of a bool array

According to the docs, `matplotlib.path.Path.contains_points()` ought to return a boolean array, but it currently returns a uint8 array. It looks like this API change was introduced in ba4016014cb4fb4927e36ce8ea429fed47dcb787. It caught me out when I tried to use the result of `contains_points()` as a boolean index to another array.