flake8: move to Python 3

Closes #33127.

Signed-off-by: FX Coudert <fxcoudert@gmail.com>

Formula/flake8.rb

@@ -5,6 +5,7 @@ class Flake8 < Formula
   homepage "http://flake8.pycqa.org/"
   url "https://gitlab.com/pycqa/flake8/repository/archive.tar.gz?ref=3.5.0"
   sha256 "97ecdc088b9cda5acfaa6f84d9d830711669ad8d106d5c68d5897ece3c5cdfda"
+  revision 1
   head "https://gitlab.com/PyCQA/flake8.git", :shallow => false
 
   bottle do
@@ -15,10 +16,10 @@ class Flake8 < Formula
     sha256 "cfbc382496c31b5c57e31ac2487d022a07673d9efd2d64cbc956d3e05c8d9afe" => :el_capitan
   end
 
-  depends_on "python@2"
+  depends_on "python"
 
   def install
-    venv = virtualenv_create(libexec)
+    venv = virtualenv_create(libexec, "python3")
     system libexec/"bin/pip", "install", "-v", "--no-binary", ":all:",
                               "--ignore-installed", buildpath
     system libexec/"bin/pip", "uninstall", "-y", name
@@ -26,6 +27,7 @@ class Flake8 < Formula
   end
 
   test do
-    system "#{bin}/flake8", "#{libexec}/lib/python2.7/site-packages/flake8"
+    xy = Language::Python.major_minor_version "python3"
+    system "#{bin}/flake8", "#{libexec}/lib/python#{xy}/site-packages/flake8"
   end
 end