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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_vm_class_for_block_number(cls, block_number: BlockNumber) -> Type['BaseVM']: """ Returns the VM class for the given block number. """

        if cls.vm_configuration is None:
            raise AttributeError("Chain classes must define the VMs in vm_configuration")

        validate_block_number(block_number)
        for start_block, vm_class in reversed(cls.vm_configuration):
            if block_number >= start_block:
                return vm_class
        else:
            raise VMNotFound("No vm available for block #{0}".format(block_number))