Error in loading the GLDv2 dataset
Hello! Thanks for the fantastic library. I have run into an error while loading the GLDv2 dataset via TFF. 

Please see the snippet below:
```
>>> import tensorflow_federated as tff
>>> dataset = tff.simulation.datasets.gldv2.load_data()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/python3.8/site-packages/tensorflow_federated/python/simulation/datasets/gldv2.py", line 396, in load_data
    qh = logging.handlers.QueueHandler(q)
AttributeError: module 'logging' has no attribute 'handlers'
```

## The root cause of this error
The error appears to be because of a missing import in `gldv2.py`. We can isolate this error to the following lines which occur within `gldv2.py`. Compare this snippet which does not work
```
>>> import logging, multiprocessing
>>> q = multiprocessing.Queue(-1)
>>> qh = logging.handlers.QueueHandler(q)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'logging' has no attribute 'handlers'
```
to this one which does
```
>>> import logging, multiprocessing
>>> import logging.handlers            # Add this import statement
>>> q = multiprocessing.Queue(-1)
>>> qh = logging.handlers.QueueHandler(q)  # works!
```

## Fixing the error
Adding an import statement `import logging.handlers` in `gldv2.py` fixes the issue -- I've tested this out locally. I can send a pull request. 

**Environment (please complete the following information):**
* OS Platform and Distribution: Ubuntu 20.04
* Python package versions (e.g., TensorFlow Federated, TensorFlow): TFF: 0.19.0 and TF: 2.5.0. The same error also occurs in the nightly version: TFF: 0.19.0.dev20210821 and TF: 2.7.0-dev20210822.
* Python version: 3.8
* Bazel version (if building from source): N/A (installed via pip)
* CUDA/cuDNN version: N/A
* What TensorFlow Federated execution stack are you using? simulation 


**Expected behavior**
I expect the data to be loaded correctly.


diff --git a/tensorflow_federated/python/simulation/datasets/gldv2.py b/tensorflow_federated/python/simulation/datasets/gldv2.py
index ac782fb4..ec76f694 100644
--- a/tensorflow_federated/python/simulation/datasets/gldv2.py
+++ b/tensorflow_federated/python/simulation/datasets/gldv2.py
@@ -15,6 +15,7 @@
 
 import collections
 import logging
+import logging.handlers
 import multiprocessing.pool
 import os
 import shutil
['tensorflow_federated/python/simulation/datasets/gldv2.py']