Can't enable the module on PHP < 7 (update requirements?)

Problem/Motivation

Trying to enable this module on D7 PHP 5.6 causes a parse error due to the usage of null coalescing operators

Steps to reproduce

Have an environment on < PHP 7 and try to enable the module

Proposed resolution

Add PHP7+ as a requirement in the main project page or/and in the readme? Or ideally via Composer?

GetAttributesAttributes.php
$this->type = $data['type'] ?? NULL;

Also

sendinblue.manage.inc
$subject = $subjects[$type] ?? '[SendinBlue]';

Yes I realise that PHP5.6 isn't ideal, long story :)

However we could do with making it clear this module requires PHP7+.