You need to indent your code an extra time before pasting to reddit, else you lose formatting. Here, I've taken my best guess at your indents:

class PythagoreanProof(Scene):
    CONFIG={
        "l_a":5/5,
        "l_b":12/5,
        "l_c":13/5,
    }
    def construct(self):
        self.wait()
        self.pre_cuadrado()
        self.pos_cuadrado()
        self.tran_pre_pos_cuadrado()
        self.wait()

    def pre_cuadrado(self):
        cuadro=Square(side_length=self.l_a+self.l_b)

As you can see, I can access the key "l_a" through self.l_a
But in this much simpler code I get the following error

class Example():
    CONFIG = {'spam':25}
    def __init__(self, value):
        self.data = value
    def display(self):
        print(self.data, self.spam)
        x=Example(2)
        x.display()