Bump protobuf from 3.5.1 to 3.15.0 in /docs/source

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.5.1 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@v3.5.1...v3.15.0)

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

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

docs/source/requirements.txt

@@ -14,7 +14,7 @@ MarkupSafe
 pathtools==0.1.2
 pipreqs==0.4.9
 port-for==0.3.1
-protobuf==3.5.1
+protobuf==3.15.0
 Pygments==2.7.4
 pytz==2017.3
 PyYAML==5.4