Bump protobuf from 3.11.3 to 3.18.3 in /python3

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.11.3 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.11.3...v3.18.3)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

python3/requirements.txt

@@ -1,3 +1,3 @@
 grpcio==1.27.2
-protobuf==3.11.3
+protobuf==3.18.3
 PyYAML==5.3.1