Window closing fix (TheAlgorithms#8625)

* The window will now remain open after the fractal is finished being drawn, and will only close upon your click.

* Update fractals/sierpinski_triangle.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: Tianyi Zheng <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

fractals/sierpinski_triangle.py

@@ -82,3 +82,4 @@ def triangle(
 
     vertices = [(-175, -125), (0, 175), (175, -125)]  # vertices of triangle
     triangle(vertices[0], vertices[1], vertices[2], int(sys.argv[1]))
+    turtle.Screen().exitonclick()