chore(deps): bump protobuf from 3.11.3 to 3.15.0

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.11.3 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@v3.11.3...v3.15.0)

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

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

requirements.txt

@@ -13,7 +13,7 @@ lru-dict==1.1.6
 multiaddr==0.0.9
 netaddr==0.7.19
 parsimonious==0.8.1
-protobuf==3.11.3
+protobuf==3.15.0
 pycryptodome==3.9.6
 pyrsistent==0.15.7
 requests==2.22.0