input: Write a python function to create a Student object and enroll in courses, then retrieve the grade.
  ```python
  student = Student("Alice", 20, "A", ["Math", "Science"])
  student.enroll_in_course("History")
  assert student.courses == ["Math", "Science", "History"]
  assert student.get_grade() == "A"
  
  student2 = Student("Bob", 21, "B", ["English"])
  student2.enroll_in_course("Art")
  assert student2.courses == ["English", "Art"]
  assert student2.get_grade() == "B"
output: class Student:
      def __init__(self, name, age, grade, courses):
          self.name = name
          self.age = age
          self.grade = grade
          self.courses = courses
  
      def enroll_in_course(self, course):
          if course not in self.courses:
              self.courses.append(course)
  
      def get_grade(self):
          return self.grade