Add python linter and formatter

Author: well1791

.flake8

@@ -0,0 +1,14 @@
+[flake8]
+exclude =
+    ./tools/*,
+    ./venv/*,
+    ./twiml/*,
+    ./guides/*,
+    ./testable_snippets/*,
+    */testable_snippets/*
+
+ignore =
+    # D100: Missing docstring in public module
+    D100,
+    # H306: imports not in alphabetical order (time, os)
+    H306,

.style.yapf

@@ -0,0 +1,3 @@
+[style]
+dedent_closing_brackets = True
+join_multiple_lines = False

install_travis_dependencies.sh

@@ -16,6 +16,7 @@ sudo apt-get install -y --force-yes gradle
 if [[ $languages == *"python"* ]]; then
   sudo apt-get install -y --force-yes python-dev python-pip
   sudo pip install --upgrade pip wheel virtualenv virtualenvwrapper --quiet
+  sudo pip install -r requirements.txt
 fi
 
 if [[ $languages == *"php"* ]]; then

requirements.txt

@@ -0,0 +1,7 @@
+configparser==3.5.0
+enum34==1.1.6
+flake8==3.4.1
+mccabe==0.6.1
+pycodestyle==2.3.1
+pyflakes==1.5.0
+yapf==0.16.3

tools/snippet-testing/language_handler/python.rb

@@ -4,6 +4,17 @@ module LanguageHandler
   class Python < BaseHandler
     LANG_CNAME = 'py'.freeze
 
+    def self.run_before_test(directory)
+      dirs = ['./venv/*', './guides/*', './testable_snippets/*']
+      exclude = dirs.map{|dir| "--exclude='#{dir}'"}
+      Dir.chdir(directory) do
+        output = `yapf #{exclude.join(' ')} -i -r . && flake8`
+        if $? != 0
+          abort(output)
+        end
+      end
+    end
+
     private
 
     def execute_command(file)