Bump protobuf from 3.12.2 to 3.18.3

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.12.2 to 3.18.3.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/compare/v3.12.2...v3.18.3)

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

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

requirements.txt

@@ -48,7 +48,7 @@ packaging==20.4
 Paginator==0.5.1
 pbr==5.4.5
 Pillow==8.2.0
-protobuf==3.12.2
+protobuf==3.18.3
 psutil==5.7.2
 pycodestyle==2.6.0
 pycparser==2.20