Bump protobuf from 3.7.0 to 3.15.0 in /python-example

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.7.0 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@v3.7.0...v3.15.0)

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

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

Author: dependabot[bot]

python-example/Pipfile

@@ -5,7 +5,7 @@ verify_ssl = true
 
 [packages]
 pitayaserver = "==0.3.0"
-protobuf = "==3.7.0"
+protobuf = "==3.15.0"
 six = "==1.12.0"
 uuid = "==1.30"