Update TFPs attrs dependency to version 21.4.0.

PiperOrigin-RevId: 455655859

Author: michaelreneer

setup.py

@@ -28,7 +28,7 @@
     license='Apache-2.0',
     install_requires=[
         'absl-py~=1.0.0',
-        'attrs~=21.2.0',
+        'attrs~=21.4.0',
         'dm-tree~=0.1.1',
         'matplotlib~=3.3.4',
         'numpy~=1.21.5',