subclass of `Box` does not unpickle correctly

## Steps to reproduce

Execute this program:

```python
import box
import pickle

class MyBox(box.Box):
...

b_in = MyBox()
b_in.a = dict()
print("in root: %s" % type(b_in))
print("in .a: %s" % type(b_in.a))

p = pickle.dumps(b_in, protocol=pickle.HIGHEST_PROTOCOL)

b_out = pickle.loads(p)
print("out root: %s" % type(b_out))
print("out .a: %s" % type(b_out.a))
```

## Expected behavior

Output:

```
in root:
in .a:
out root:
out .a:
```

## Actual behavior

Output:

```
in root:
in .a:
out root:
out .a:
```

i.e., the type of key `a` is `box.box.Box` when it should be `__main__.MyBox`.

## Comments

1. I believe the problem lies in this line:

https://github.com/cdgriffith/Box/blob/a4c10e977b574114613431394b30412b50aaacce/box/box.py#L236

where `Box` should insted be `cls`.

2. Instantiating `b_in` with `box_class=MyBox` does not help, I assume because there is something about class instantiation I don’t understand. However it did seem like it should work.

3. This does, however, work around the bug:

```python
class MyBox(box.Box):
def __new__(cls, *args, **kwargs):
kwargs["box_class"] = MyBox
return super().__new__(cls, *args, **kwargs)
```

Run the reproduction program from the issue, then inspect box/box.py around line 236 where nested boxes are constructed during unpickling. Confirm that a Box subclass remains the type of nested values after pickle.dumps and pickle.loads; done means the reported output shows MyBox for both the root and key a.

Written by the indexing model from the issue text.