input: Write a python function to initialize a bookstore, add books, and get a book by its title.
  assert bookstore.get_book_by_title('Python 101').title == 'Python 101'
  assert bookstore.get_book_by_title('Data Science Handbook').title == 'Data Science Handbook'
  assert bookstore.get_book_by_title('Nonexistent Book') == None
output: class Book:
      def __init__(self, title, author):
          self.title = title
          self.author = author
  
  class Bookstore:
      def __init__(self):
          self.books = []
  
      def add_book(self, book):
          self.books.append(book)
          
      def get_book_by_title(self, title):
          for book in self.books:
              if book.title == title:
                  return book
          return None
  
  # Usage
  bookstore = Bookstore()
  bookstore.add_book(Book('Python 101', 'Author A'))
  bookstore.add_book(Book('Data Science Handbook', 'Author B'))
  
  assert bookstore.get_book_by_title('Python 101').title == 'Python 101'
  assert bookstore.get_book_by_title('Data Science Handbook').title == 'Data Science Handbook'
  assert bookstore.get_book_by_title('Nonexistent Book') == None