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

Release-As: 1.20.0

Closes #568
diff --git a/docs/reference/google.auth.transport.rst b/docs/reference/google.auth.transport.rst
index 48e2e05..8921863 100644
--- a/docs/reference/google.auth.transport.rst
+++ b/docs/reference/google.auth.transport.rst
@@ -10,7 +10,9 @@
 ----------
 
 .. toctree::
+   :maxdepth: 4
 
    google.auth.transport.grpc
+   google.auth.transport.mtls
    google.auth.transport.requests
    google.auth.transport.urllib3