Bump protobuf from 3.12.2 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.12.2 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.12.2...v3.15.0)

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

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

requirements.txt

@@ -29,7 +29,7 @@ opt-einsum==3.2.1
 packaging==20.4
 pandas==1.0.5
 pluggy==0.13.1
-protobuf==3.12.2
+protobuf==3.15.0
 py==1.10.0
 pyasn1==0.4.8
 pyasn1-modules==0.2.8