chore: add yoshi-python as additional code owner (googleapis#612)

Author: parthea

.github/CODEOWNERS

@@ -7,5 +7,5 @@
 
 # The api-spanner-python team is the default owner for anything not
 # explicitly taken by someone else.
-*                               @googleapis/api-spanner-python
-/samples/                       @googleapis/api-spanner-python @googleapis/python-samples-owners
+*                               @googleapis/api-spanner-python @googleapis/yoshi-python
+/samples/                       @googleapis/api-spanner-python @googleapis/python-samples-owners