Avoid deprecated attrs feature `convert`

https://github.com/pytest-dev/pytest/commit/37793d4cdbf71f597a3c69c827e0c010528d53a5 introduced a use of the `convert` keyword argument to `attr.ib`.  This is deprecated in attrs:

> DeprecationWarning: The `convert` argument is deprecated in favor of `converter`.  It will be removed after 2019/01.                                 

The replacement should be used instead.
