chore: release 1.9.0 (#410)

diff --git a/setup.py b/setup.py
index d98e89c..f4a8b17 100644
--- a/setup.py
+++ b/setup.py
@@ -30,7 +30,7 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.8.2"
+version = "1.9.0"
 
 setup(
     name="google-auth",