Bump protobuf from 3.14.0 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.14.0 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@v3.14.0...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

@@ -33,7 +33,7 @@ nose==1.3.7
 numpy==1.19.4
 pandas==1.1.4
 Pillow==8.0.1
-protobuf==3.14.0
+protobuf==3.15.0
 pyparsing==2.4.7
 python-dateutil==2.8.1
 pytz==2020.4