input: Write a python function to manage a grocery shop's inventory.
  assert grocery_shop.get_stock('apples') == 5
  assert grocery_shop.get_stock('bananas') == 10
  assert grocery_shop.get_stock('oranges') == 3
  assert grocery_shop.get_stock('pears') == None
output: class GroceryShop:
      def __init__(self):
          self.inventory = {}
      
      def add_stock(self, item, quantity):
          if item in self.inventory:
              self.inventory[item] += quantity
          else:
              self.inventory[item] = quantity
              
      def get_stock(self, item):
          return self.inventory.get(item, None)
  
  # Creating an instance of GroceryShop and adding stock
  grocery_shop = GroceryShop()
  grocery_shop.add_stock('apples', 5)
  grocery_shop.add_stock('bananas', 10)
  grocery_shop.add_stock('oranges', 3)