chore: add default CODEOWNERS (#93)

Author: busunkim96

packages/google-cloud-texttospeech/.github/CODEOWNERS

@@ -4,6 +4,9 @@
 # For syntax help see:
 # 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
+
 
 # The python-samples-owners team is the default owner for samples
 /samples/**/*.py                       @telpirion @sirtorry @googleapis/python-samples-owners