Bump protobuf from 3.15.8 to 3.18.3 (#2)

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.8 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/v3.15.8...v3.18.3)

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

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

@@ -26,7 +26,7 @@ pathtools==0.1.2
 Pillow==9.0.1
 pooch==1.3.0
 promise==2.3
-protobuf==3.15.8
+protobuf==3.18.3
 psutil==5.8.0
 pycparser==2.20
 pyparsing==2.4.7