Add sso to list of known schemes for relative URLs

repo already special-cases sso:// URLs to behave similarly to https://
and rpc:// elsewhere in repo, but it forgot to do so here.

Noticed when trying to use relative URLs in a manifest obtained using
an sso:// URL.

Change-Id: Ia11469a09bbd6e444dbc4f22c82f9bbe9f5fd083

manifest_xml.py

@@ -40,8 +40,18 @@
 LOCAL_MANIFESTS_DIR_NAME = 'local_manifests'
 
 # urljoin gets confused if the scheme is not known.
-urllib.parse.uses_relative.extend(['ssh', 'git', 'persistent-https', 'rpc'])
-urllib.parse.uses_netloc.extend(['ssh', 'git', 'persistent-https', 'rpc'])
+urllib.parse.uses_relative.extend([
+    'ssh',
+    'git',
+    'persistent-https',
+    'sso',
+    'rpc'])
+urllib.parse.uses_netloc.extend([
+    'ssh',
+    'git',
+    'persistent-https',
+    'sso',
+    'rpc'])
 
 class _Default(object):
   """Project defaults within the manifest."""