Improve compatibility between turtle.FormattedTuple and stdlib's turtle.Vec2D (turtle star)

[m-aciek]

Turtle module docs intro contains following script for drawing turtle star:

color('red')
fillcolor('yellow')

begin_fill()
while True:
    forward(200)
    left(170)
    if abs(pos()) < 1:
        break

end_fill()
done()

It draws correctly with tkinter, but abs(pos()) < 1 fails with TypeError: Bad operand type for abs(): 'FormattedTuple' for Brython (and Basthon).

Fix by copying over __abs__ method implementation for stdlib. __abs__ method returns the absolute value of the point.

[m-aciek]

With the same change in Basthon turtle source I was able to draw the SVG:

cc @bearney74

[m-aciek]

Slightly related: I also opened a PR to simplify this docs fragment to not use the abs(), led by motivation of easier reader comprehension.

[m-aciek]

cc @casatir -- the same issue applies to Basthon turtle, but I don't have access to post a pull request for it.

[PierreQuentel]

Thanks for the PR Maciej, and sorry for the late reaction, August is more dedicated to holidays and family !

[m-aciek]

Thank you! No worries. Just to mention in the meantime we've simplified code example in the turtle tutorial.