Incompatible with Pynamodb 6.x

With pynamodb 6.0.1, I got a test failure:

```
________________________________ test_exception ________________________________

def test_exception():
class SampleModel(Model):
class Meta:
region = 'us-west-2'
table_name = 'mytable'

sample_attribute = UnicodeAttribute(hash_key=True)

try:
SampleModel.describe_table()
except Exception:
pass

subsegments = xray_recorder.current_segment().subsegments
assert len(subsegments) == 1
subsegment = subsegments[0]
assert subsegment.name == 'dynamodb'
> assert len(subsegment.subsegments) == 0
E assert 1 == 0
E + where 1 = len([])
E + where [] = .subsegments

tests/ext/pynamodb/test_pynamodb.py:48: AssertionError
```