[ADD] Flake8 configuration file

Using 'line-length=80' and flake8-bugbear's B950 warning, I respect
black's formatting of 88 columns max.

The T4 category is related to mypy lints, using the 'flake8-mypy`
plugin.

The max complexity is supposed to make you write shorter functions with
less complexity, using the cyclomatic complexity heuristic.

This file is almost directly taken from black's own flake8
configuration, because I'm using it as a linter and I mostly agreed with
its settings.

flake8/.config/flake8

@@ -0,0 +1,22 @@
+[flake8]
+ignore =
+    # Whitespace before a ':' in slices
+    E203,
+    # Line over 80 columns
+    E501,
+    # Line break before a binary operator
+    W503
+
+# Up-to 88 thanks to B950 violation
+max-line-length = 80
+
+# Cyclomatic complexity for functions
+max-complexity = 18
+
+select =
+    # Built-in violations (complexity, whitespace, and others)
+    C,E,F,W,
+    # Violations from flake8-mypy
+    T4,
+    # Violations from flake8-bugbear, with opinionated warnings
+    B,B9

install.sh

@@ -5,6 +5,7 @@ stow -t ~ X
 stow -t ~ bash
 stow -t ~ dunst
 stow -t ~ emacs
+stow -t ~ flake8
 stow -t ~ git
 stow -t ~ i3
 stow -t ~ i3blocks