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

setup.cfg

@@ -2,6 +2,111 @@
 # black's settings are in pyproject.toml => [tool.black]
 
 
+[flake8]
+# Include error classes only explicitly
+# to avoid forward compatibility issues.
+select =
+    # =============
+    # flake8's base
+    # =============
+    # mccabe => cyclomatic complexity
+    C901,
+    # pycodestyle => PEP8 compliance
+    E, W,
+    # pyflakes => basic errors
+    F4, F5, F6, F7, F8, F9
+    # ========================
+    # wemake-python-styleguide
+    # Source: https://wemake-python-stylegui.de/en/latest/pages/usage/violations/index.html
+    # ========================
+    WPS1, WPS2, WPS3, WPS4, WPS5, WPS6,
+    # darglint => docstring matches implementation
+    DAR0, DAR1, DAR2, DAR3, DAR4, DAR5,
+    # flake8-bandit => common security issues
+    S1, S2, S3, S4, S5, S6, S7,
+    # flake8-broken-line => no \ to end a line
+    N400,
+    # flake8-bugbear => opinionated bugs and design flaws
+    B0, B3, B9,
+    # flake8-commas => better comma placements
+    C8,
+    # flake8-comprehensions => better comprehensions
+    C4,
+    # flake8-debugger => no debugger usage
+    T100,
+    # flake8-docstrings => PEP257 compliance
+    D1, D2, D3, D4,
+    # flake8-eradicate => no commented out code
+    E800,
+    # flake8-isort => isort would make changes
+    I0,
+    # flake8-rst-docstrings => valid rst in docstrings
+    RST2, RST3, RST4,
+    # flake8-string-format => unify usage of str.format()
+    P1, P2, P3,
+    # flake8-quotes => use double quotes everywhere (complying with black)
+    Q0,
+    # pep8-naming
+    N8,
+    # =====
+    # other
+    # =====
+    # flake8-annotations => enforce type checking for functions
+    ANN0, ANN2, ANN3,
+    # flake8-black => complain if black would make changes
+    BLK1, BLK9,
+    # flake8-expression-complexity => not too many expressions at once
+    ECE001,
+
+# By default, flake8 ignores some errors.
+# Instead, do not ignore anything.
+ignore =
+
+# If --ignore is passed on the command
+# line, still ignore the following:
+extend-ignore =
+    # Comply with black's style.
+    # Source: https://github.com/psf/black/blob/master/docs/compatible_configs.md#flake8
+    E203, W503,
+    # f-strings are ok.
+    WPS305,
+
+per-file-ignores =
+    noxfile.py:
+        # TODO (isort): Check if still too many module members.
+        WPS202,
+        # TODO (isort): Remove after simplifying the nox session "lint".
+        WPS213,
+        # No overuse of string constants (e.g., '--version').
+        WPS226,
+
+# Explicitly set mccabe's maximum complexity to 10 as recommended by
+# Thomas McCabe, the inventor of the McCabe complexity, and the NIST.
+# Source: https://en.wikipedia.org/wiki/Cyclomatic_complexity#Limiting_complexity_during_development
+max-complexity = 10
+
+# Comply with black's style.
+# Source: https://github.com/psf/black/blob/master/docs/the_black_code_style.md#line-length
+max-line-length = 88
+
+# Preview the code lines that cause errors.
+show-source = true
+
+# ===================================
+# wemake-python-styleguide's settings
+# ===================================
+allowed-domain-names =
+    result,
+min-name-length = 3
+max-name-length = 40
+# darglint
+strictness = long
+# flake8-docstrings
+docstring-convention = google
+# flake8-eradicate
+eradicate-aggressive = true
+
+
 [isort]
 atomic = true
 case_sensitive = true
@@ -24,3 +129,42 @@ use_parentheses = true
 # Source: https://google.github.io/styleguide/pyguide.html#313-imports-formatting
 force_single_line = true
 single_line_exclusions = typing
+
+
+[mypy]
+cache_dir = .cache/mypy
+
+[mypy-nox.*]
+ignore_missing_imports = true
+
+
+[pylint.FORMAT]
+# Comply with black's style.
+max-line-length = 88
+
+[pylint.MESSAGES CONTROL]
+disable =
+    # We use TODO's to indicate locations in the source base
+    # that must be worked on in the near future.
+    fixme,
+    # Comply with black's style.
+    bad-continuation, bad-whitespace,
+    # =====================
+    # flake8 de-duplication
+    # Source: https://pylint.pycqa.org/en/latest/faq.html#i-am-using-another-popular-linter-alongside-pylint-which-messages-should-i-disable-to-avoid-duplicates
+    # =====================
+    # mccabe
+    too-many-branches,
+    # pep8-naming
+    bad-classmethod-argument, bad-mcs-classmethod-argument,
+    invalid-name, no-self-argument,
+    # pycodestyle
+    bad-indentation, bare-except, line-too-long, missing-final-newline,
+    multiple-statements, trailing-whitespace, unnecessary-semicolon, unneeded-not,
+    # pydocstyle
+    missing-class-docstring, missing-function-docstring, missing-module-docstring,
+    # pyflakes
+    undefined-variable, unused-import, unused-variable,
+
+[pylint.REPORTS]
+score = no