Remove tox's system site packages
type_index for mediafile images defaults to "other"

The `Image.type_index` property is only used for tags that require an integer
to store the cover type like ID3v2. If the type of an image is not set we let
`type_index` return the integer for an "other" image. Other than a default
"front" value, this makes sure we do not inadvertidly add an image that will
be displayed in a player.