Activate LOTS of flake8 plugins to enforce code hygiene

Selection criteria:

* Must be useful, even if many warnings
  For example: Plugins related to annotations. As @warsaw raised in
  aio-libs#128, it's a good idea to type-annotate as much as possible.

* If not simple, must be maintained
  "maintained" here is overly-simplified to mean only "master has an
  update in 2020 or later".
  Some plugins have not received updates for a long time, but their
  logic are simple and really does not need updates, so those can also
  be included on a case-by-case basis.

* Will not need significant change in logic
  Adding annotations, removing temp vars used just for return values,
  those are not logic changes. So even if lots of instances need to be
  fixed, that is okay.
  Plugins that need a restructurization of the logic flow (e.g.,
  "flake8-cognitive-complexity") should not be included; or, if
  included, should not be activated. At least not until the next Epic.

setup.cfg

@@ -64,4 +64,39 @@ source-dir = aiosmtpd/docs
 [flake8]
 jobs = 1
 max-line-length = 88
-ignore = E123, E133, W503, W504, W293, E203
+# "E,F,W,C90" are flake8 defaults
+# For others, take a gander at tox.ini to see which prefix provided by who
+select = E,F,W,C90,C4,MOD,JS,PIE,PT,SIM,ECE,C801,DUO,TAE,ANN,G,YTT,N400
+ignore =
+    # black conflicts with E123 & E133
+    E123
+    E133
+    # W503 conflicts with PEP8...
+    W503
+    # W293 is a bit too noisy. Many files have been edited using editors that do not remove spaces from blank lines.
+    W293
+    # Sometimes spaces around colons improve readability
+    E203
+    # Sometimes we prefer the func()-based creation, not literal, for readability
+    C408
+    # Sometimes we need to catch Exception broadly
+    PIE786
+    # We don't really care about pytest.fixture vs pytest.fixture()
+    PT001
+    # Good idea, but too many changes. Remove this in the future, and create separate PR
+    PT004
+    # Sometimes exception needs to be explicitly raised in special circumstances, needing additional lines of code
+    PT012
+    # I still can't grok the need to annotate "self" or "cls" ...
+    ANN101
+    ANN102
+    # I don't think forcing annotation for *args and **kwargs is a wise idea...
+    ANN002
+    ANN003
+no-accept-encodings = True
+copyright-check = True
+# The number below was determined empirically by bisecting from 100 until no copyright-unnecessary files appear
+copyright-min-file-size = 44
+copyright-author = The aiosmtpd Developers
+max-annotations-complexity = 4
+mypy-init-return = True

tox.ini

@@ -56,13 +56,51 @@ basepython = python3
 envdir = {toxworkdir}/qa
 commands =
     python housekeep.py prep
+    # The next line lists enabled plugins
+    python -m flake8 --version
     python -m flake8 aiosmtpd setup.py housekeep.py release.py
     check-manifest -v
     pytest -v --tb=short aiosmtpd/qa
+# Snippets of letters above flake8- plugins are tests that need to be "select"-ed in flake8 config to
+# activate the respective plugins. If no snippet is given, that means the plugin is always active
 deps =
     colorama
     flake8
     flake8-bugbear
+    flake8-builtins
+    flake8-coding
+    # C4
+    flake8-comprehensions
+    # JS
+    flake8-multiline-containers
+    # PIE
+    flake8-pie
+    # MOD
+    flake8-printf-formatting
+    # PT
+    flake8-pytest-style
+    # SIM
+    flake8-simplify
+    # Cognitive Complexity looks like a good idea, but to fix the complaints... it will be an epic effort.
+    # So we disable it for now and reenable when we're ready, probably just before 2.0
+    # # CCR
+    # flake8-cognitive-complexity
+    # ECE
+    flake8-expression-complexity
+    # C801
+    flake8-copyright
+    # DUO
+    dlint
+    # TAE
+    flake8-annotations-complexity
+    # ANN
+    flake8-annotations
+    # G
+    flake8-logging-format
+    # YTT
+    flake8-2020
+    # N400
+    flake8-broken-line
     pytest
     check-manifest