build(deps): bump protobuf from 3.15.5 to 3.15.6

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.5 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.5...v3.15.6)

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

requirements.txt

@@ -7,7 +7,7 @@ mccabe==0.6.1
 mysql-connector-python==8.0.23
 prettytable==2.1.0
 prompt-toolkit==3.0.16
-protobuf==3.15.5
+protobuf==3.15.6
 pycodestyle==2.6.0
 Pygments==2.8.1
 pylint==2.7.2