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]>

Author: dependabot[bot]

dev-requirements.txt

@@ -1,4 +1,4 @@
 PyNaCl==1.4.0
-protobuf==3.12.2
+protobuf==3.15.0
 grpcio_tools==1.21.1
 six==1.12.0