Bump protobuf from 3.8.0 to 3.18.3 in /integration-tests/python

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.8.0 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@v3.8.0...v3.18.3)

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

Signed-off-by: dependabot[bot] <support@github.com>

integration-tests/python/requirements.txt

@@ -6,7 +6,7 @@ grpcio==1.21.1
 importlib-metadata==0.17
 more-itertools==7.0.0
 pluggy==0.12.0
-protobuf==3.8.0
+protobuf==3.18.3
 py==1.10.0
 pyasn1==0.4.5
 pyasn1-modules==0.2.5