Add absl as an explicit dependency. Without it, numpy backend does not work

PiperOrigin-RevId: 396901654

Author: csuter

setup.py

@@ -28,6 +28,7 @@
 from version import __version__  # pylint: disable=g-import-not-at-top
 
 REQUIRED_PACKAGES = [
+    'absl-py',
     'six >= 1.10.0',
     'numpy >= 1.13.3',
     'decorator',