RationalNumber Class

Your task is to define a RationalNumber class in Python. This class should be used to represent a rational number, a number that can be expressed as the quotient of two integers (numerator and denominator).

The RationalNumber class should have the following characteristics:

• The class should have two private integer attributes, the numerator and the denominator of the rational number.

• The class should define the magic methods __add__, __sub__, __mul__, and __truediv__ to add, subtract, multiply, and divide two rational numbers, respectively.

• Addition, subtraction, multiplication, and division operations should be performed in a way that's mathematically accurate for rational numbers, with the result being another instance of RationalNumber.

• The class should reduce the rational number to its simplest form by dividing the numerator and denominator by their greatest common divisor (GCD).

• The class should implement the __str__ magic method that should return a string <numerator> / <denominator> in its simplest form.