Default Box fails to handle singleton instances of itself

It appears that trying to use Default Box in a singleton pattern breaks it. See below for code and traceback

```python
from box import Box

_test_box = None

class TestBox(Box):
def __new__(cls, *args, **kwargs):
global _test_box
if _test_box is None:
_test_box = super().__new__(cls, *args, **kwargs)
return _test_box

def __init__(self, *args, **kwargs):
super().__init__(default_box=True, *args, **kwargs)
self.__test_method()

def __test_method(self):
_this_should_not_fail = self.env

TestBox()
```

Traceback is as follows
```python
Traceback (most recent call last):
File "/home/miversen/git-local/transcoder/venv/lib/python3.8/site-packages/box/box.py", line 488, in __getitem__
return super().__getitem__(item)
KeyError: 'env'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/miversen/git-local/transcoder/venv/lib/python3.8/site-packages/box/box.py", line 514, in __getattr__
value = self.__getitem__(item, _ignore_default=True)
File "/home/miversen/git-local/transcoder/venv/lib/python3.8/site-packages/box/box.py", line 509, in __getitem__
raise BoxKeyError(str(err)) from _exception_cause(err)
box.exceptions.BoxKeyError: "'env'"

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/miversen/git-local/transcoder/venv/lib/python3.8/site-packages/box/box.py", line 516, in __getattr__
value = object.__getattribute__(self, item)
AttributeError: 'TestBox' object has no attribute 'env'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "test.py", line 19, in
c1 = TestBox()
File "test.py", line 14, in __init__
self.__test_method()
File "test.py", line 17, in __test_method
_this_should_not_fail = self.env
File "/home/miversen/git-local/transcoder/venv/lib/python3.8/site-packages/box/box.py", line 529, in __getattr__
return self.__get_default(item, attr=True)
File "/home/miversen/git-local/transcoder/venv/lib/python3.8/site-packages/box/box.py", line 426, in __get_default
value = self._box_config["box_class"](**self.__box_config())
File "test.py", line 13, in __init__
super().__init__(default_box=True, *args, **kwargs)
KeyError: 'default_box'
```

Environment Details

- Python3: Python 3.8.10
- OS: Ubuntu 20.04.3 LTS
- Kernel: 5.11.0-27-generic