The commit replaces the `Man` model used in tests with a `Human` model. It
also replaces the existing `Human` model with a `SuperHuman` model
inheriting from `Human`.

While this may seem like a cosmetic change, I see it as more of an
inclusivity change. I think it makes sense for a number of reasons:

* Prior to this commit the `Human` model inherited from `Man`. At best
  this makes no sense (it should be the other way around). At worst it
  is offensive and harmful to the community.
* It doesn't seem inclusive to me to have exclusively male-gendered
  examples in the codebase.
* There is no particular reason for these examples to be gendered.
* `man` is hard to grep for, since it also matches `many, manager,
  manual, etc`

For the most part this is a simple search and replace. The one exception
to that is that I had to add the table name to the model so we could use
"humans" instead of "humen".