创建一个名为 `BankAccount` 的类，包含以下属性：`balance` 和 `account_number`。创建一个类方法 `create_account()`，用于创建一个新的银行账户。请给出使用示例。

class BankAccount:
    account_number = 0

    def __init__(self, balance):
        self.balance = balance
        BankAccount.account_number += 1
        self.account_number = BankAccount.account_number

    @classmethod
    def create_account(cls, initial_balance):
        return cls(initial_balance)

account1 = BankAccount.create_account(1000)
print(f"Account number {account1.account_number} has a balance of {account1.balance}")
account2 = BankAccount.create_account(5000)
print(f"Account number {account2.account_number} has a balance of {account2.balance}")

输出：

Account number 1 has a balance of 1000
Account number 2 has a balance of 5000