Bump protobuf from 3.11.3 to 3.18.3

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]>

Author: dependabot[bot]

requirements.txt

@@ -5,7 +5,7 @@ future==0.18.2
 gym==0.10.0
 idna==2.9
 numpy==1.18.4
-protobuf==3.11.3
+protobuf==3.18.3
 pyglet==1.5.0
 requests==2.23.0
 scipy==1.4.1