docs(README): state that Sbroad is an executor for `cartridge`-based cluster, because `vshard` cluster is required but not enough

1. Review `Pipfile`:
  - Remove unused `filelock`;
  - Install `mypy` - static type checker for Python.
2. Add new command `pipenv run lint`.
3. Enable `mypy` in CI. Fix reported errors in `test_basics.py`.
4. Renew readme.