chore: release 1.21.3 (#607)

diff --git a/setup.py b/setup.py
index ea43594..1c3578f 100644
--- a/setup.py
+++ b/setup.py
@@ -33,7 +33,7 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.21.2"
+version = "1.21.3"
 
 setup(
     name="google-auth",