如何在kivymd按钮中对齐文本？

from kivymd.app import MDApp
from kivymd.uix.button import MDRectangleFlatButton
from kivy.lang import Builder

KV = '''
BoxLayout:
    orientation: 'vertical'

    MDRectangleFlatButton:
        text: "Left Aligned Text"
        text_align: "left"

    MDRectangleFlatButton:
        text: "Center Aligned Text"
        text_align: "center"

    MDRectangleFlatButton:
        text: "Right Aligned Text"
        text_align: "right"
'''

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

MyApp().run()