Bump protobuf from 3.9.1 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.9.1 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@v3.9.1...v3.15.0)

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

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

Author: dependabot[bot]

requirements.txt

@@ -13,7 +13,7 @@ more-itertools==5.0.0
 packaging==19.1
 pathlib2==2.3.4
 pluggy==0.13.0
-protobuf==3.9.1
+protobuf==3.15.0
 py==1.10.0
 pyparsing==2.4.2
 pytest==5.1.2