Fix undefined variable in `py_set.js`
Fix undefined variable in `py_set.js`

This would trigger a NameError whenever a set was created.

For instance, in `www/console.html`:

```
>>> set()

NameError: _b_ is not defined
  module '$exec_49' line 1
set()
```

brython-dev/brython,Hasimir/brython,kikocorreoso/brython,Hasimir/brython,jonathanverner/brython,brython-dev/brython,kikocorreoso/brython,Hasimir/brython,kikocorreoso/brython,jonathanverner/brython,jonathanverner/brython,jonathanverner/brython,brython-dev/brython,Hasimir/brython