Bump protobuf from 3.6.1 to 3.18.3

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.6.1 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@v3.6.1...v3.18.3)

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

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

requirements.txt

@@ -34,7 +34,7 @@ pbr==5.1.2
 pika==0.13.0
 ply==3.10
 prompt-toolkit==2.0.9
-protobuf==3.6.1
+protobuf==3.18.3
 pycparser==2.19
 pyfakefs==3.5.7
 Pygments==2.3.1