input: Write a python function to set and get values in a Context class.
  assert context.set('age', 25) is None
  assert context.get('age') == 25
  assert context.get('name') == 'John'
  context.clear()
  assert context.get('name') is None
  assert context.get('age') is None
output: class Context:
      def __init__(self):
          self.data = {}
  
      def set(self, key, value):
          self.data[key] = value
  
      def get(self, key):
          return self.data.get(key, None)
  
      def clear(self):
          self.data.clear()