Bump protobuf from 3.15.0 to 3.18.3

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.0 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@v3.15.0...v3.18.3)

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

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

Author: dependabot[bot]

requirements.txt

@@ -18,7 +18,7 @@ idna==2.8
 msgpack==0.6.2
 Pillow==8.3.2
 # pkg-resources==0.0.0
-protobuf==3.15.0
+protobuf==3.18.3
 pyasn1==0.4.7
 pyasn1-modules==0.2.7
 pytz==2019.3