dynamodb module: add backward-compatible support for Python 3.3+

There's one integration test left failed in Python 3, and I am not sure what is the correct way to fix it.

```
FAIL: test_lossy_float_conversion (tests.integration.dynamodb.test_layer2.DynamoDBLayer2Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/felix/projects/boto/tests/integration/dynamodb/test_layer2.py", line 470, in test_lossy_float_conversion
    self.assertNotEqual(1.12345678912345, retrieved)
nose.proxy.AssertionError: 1.12345678912345 == 1.12345678912345
```

The problem is, in Python 3, calling str() on a float is not as lossy as Python 2.x. I'm not sure if the LossyFloatDynamizer should always round the float to the same point, or the dynamizer should be kept as-is and we should fix tests to fit Python 3.