chore: add Sijun and Silvano as CODEOWNERS (#813)

This ensures they are automatically tagged in PRs for reviews.

.github/CODEOWNERS

@@ -5,7 +5,7 @@
 # https://help.github.com/en/github/creating-cloning-and-archiving-repositories/about-code-owners#codeowners-syntax
 
 # The @googleapis/yoshi-python is the default owner for changes in this repo
-*               @googleapis/yoshi-python
+*               @arithmetic1728 @silvolu @googleapis/yoshi-python
 
 # The python-samples-reviewers team is the default owner for samples changes
-/samples/  @googleapis/python-samples-owners
+/samples/  @googleapis/python-samples-owners