chore: release 1.13.0 (#477)

diff --git a/setup.py b/setup.py
index 8ae5e09..0930fd8 100644
--- a/setup.py
+++ b/setup.py
@@ -30,7 +30,7 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.12.0"
+version = "1.13.0"
 
 setup(
     name="google-auth",