Clarify license for `scripts/internal`

[stefan6419846]

The files in https://github.com/giampaolo/pyftpdlib/tree/master/scripts/internal state: https://github.com/giampaolo/pyftpdlib/blob/6d50bf7c5cd69b9827981b53233995ded5db4ad9/scripts/internal/generate_manifest.py#L3-L5 The general project license is MIT, so I think the term BSD-style license is a bit misleading and probably should be replaced.