Rounded triangle (one rounded corner)

main.py

from kivy.app import App
from kivy.lang import Builder


KV = """
Widget
    size_hint: None, None
    size: 200,200
    radius: [0,0,0,30]

    pos: 100,100

    canvas:
        StencilPush

        Color:
            rgba: 0.4,0.3,0.5,1

        RoundedRectangle
            size: self.size
            pos: self.pos
            radius: self.radius

        Triangle
            points: (self.x, self.top, self.right, self.top, self.right, self.y)

        StencilUse


        RoundedRectangle
            size: self.size
            pos: self.pos
            radius: self.radius

        StencilPop

"""


class MyApp(App):
    def build(self):
        return Builder.load_string(KV)


MyApp().run()

demo png:
https://imgur.com/a/p7UMwuA

not user friendly, just an example of how to use Stencil to make that shape