input: Write a python function to create a BankAccount class that can deposit, withdraw and return the current balance as a string.
  assert str(BankAccount(1000)) == "Current balance is 1000"
  assert BankAccount(1000).deposit(500) == 1500
  assert BankAccount(1000).withdraw(300) == 700
output: class BankAccount:
      def __init__(self, initial_balance):
          self.balance = initial_balance
  
      def deposit(self, amount):
          self.balance += amount
          return self.balance
  
      def withdraw(self, amount):
          self.balance -= amount
          return self.balance
  
      def __str__(self):
          return f"Current balance is {self.balance}"