Updates TFF's attrs dependency to 21.4.0 for compatibility with TF Privacy 0.8.1.
# Requirements for TensorFlow Federated development. If you add a dependency
# here and the dependency is required by the Python package, also add it to
# https://github.com/tensorflow/federated/blob/main/tensorflow_federated/tools/development/setup.py.
-attrs~=21.2.0
+attrs~=21.4.0
cachetools~=3.1.1
dm-tree~=0.1.7
farmhashpy~=0.4.0
