constraint --> constraints

[dudung]

I encounter a problem as executing intro7.py

AttributeError                            Traceback (most recent call last)
Input In [3], in <cell line: 10>()
      8 body.position = (350, 100)
      9 circle = pymunk.Circle(body, radius=20)
---> 10 joint = pymunk.constraint.PinJoint(space.static_body, body, (200, 200))
     12 space.add(body, circle, joint)
     14 App().run()

AttributeError: module 'pymunk' has no attribute 'constraint'

which I believe that the examples in introduction that containt

joint = pymunk.constraint.PinJoint()

should be corrected to

joint = pymunk.constraints.PinJoint()

as in pymunk/constraints, since it is missing the s.

The intro8.py and intro9.py must also be corrected.

I have submitted a pull request for this.