Add a code linting tool chain

- use flake8 as the main and pylint as the auxiliary linter
- install flake8 with the following plug-ins:
  + flake8-annotations => enforce type annotations for functions/classes
  + flake8-black => ensure black would not make any changes
  + flake8-expression-complexity
  + wemake-python-styleguide, which packages the following:
    * darglint         * flake8-bandit         * flake8-broken-line
    * flake8-bugbear   * flake8-commas         * flake8-comprehensions
    * flake8-debugger  * flake8-docstrings     * flake8-eradicate
    * flake8-isort     * flake8-rst-docstrings * flake8-string-format
    * flake8-quotes    * pep8-naming
- configure flake8 & friends in a rather explicit and strict way
- isort needed to be downgraded to ^4.3.21 due to a conflict with
  pylint and wemake-python-styleguide:
  + provide TODO's to remove the parts that "fix" isort
- use mypy for static type checking
- add a nox session "lint" that runs flake8, mypy, and pylint
- lint all source files