feat(packaging): add support for Python 3.8 (#569)

Release-As: 1.20.0

Closes #568
diff --git a/docs/reference/google.auth.rst b/docs/reference/google.auth.rst
index f6ea073..cfcf703 100644
--- a/docs/reference/google.auth.rst
+++ b/docs/reference/google.auth.rst
@@ -10,6 +10,7 @@
 -----------
 
 .. toctree::
+   :maxdepth: 4
 
    google.auth.compute_engine
    google.auth.crypt
@@ -19,6 +20,7 @@
 ----------
 
 .. toctree::
+   :maxdepth: 4
 
    google.auth.app_engine
    google.auth.credentials