chore(deps): update dependency protobuf to v3.15.5 (#976)

Co-authored-by: Jeffrey Rennie <rennie@google.com>

Author: renovate-bot

requirements.txt

@@ -5,7 +5,7 @@ requests
 pyyaml
 jinja2
 deprecation
-protobuf==3.15.2
+protobuf==3.15.5
 watchdog
 
 # https://github.com/googleapis/gapic-generator/issues/3334