chore: release 1.20.0 (#567)

diff --git a/setup.py b/setup.py
index 6a70fca..5b192b1 100644
--- a/setup.py
+++ b/setup.py
@@ -33,7 +33,7 @@
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
 
-version = "1.19.2"
+version = "1.20.0"
 
 setup(
     name="google-auth",