Support linting; Add plugin-venv test

[FabijanC]

Usage

• This PR contains no usage changes.

Dev

• Adds a linting step to the CI/CD workflow:
  • Performed with pylint and configured through .pylintrc.
  • Make necessary changes to conform to the standards set by the linter.
• Add testing with plugin using a venv:
  • So far only the dockerized plugin version was tested.
  • Dockerized and venv test are specified as separate steps in job test.
  • Rely on hardhat's --config option for specifying which config file to use.
• Instead of running all tests in starknet-hardhat-example, run only quick-test.ts.