LGTM.com - false positive - may not be initialized properly as subclass.init is not called

**Description of the false positive**

In some cases with multiple inheritance specifying explicity the subclasses constructors to be called is indeed something that has to be done. But there is also cases, such as in our case with Mixins, or if all subclasses are designed for cooperative inheritance where it's fine.

An SO answer that explains that nicely is here: https://stackoverflow.com/a/50465583/110395

**URL to the alert on the project page on LGTM.com**

https://lgtm.com/projects/g/rotki/rotki/snapshot/54e0a044f538483e949bd3ee4257951a3257bb57/files/rotkehlchen/chain/ethereum/modules/nfts.py?sort=name&dir=ASC&mode=heatmap#xa5976187fed4d9c9:1