python_dependency: fixes, features, cleanup.

- PythonDependency now implies Python 2.7
- PythonDependency now uses brewed Python for bottling
- Use double-quotes everywhere

Closes Homebrew/homebrew#27112.

Library/Homebrew/requirements/python_dependency.rb

-require 'requirement'
+require "language/python"
 
 class PythonDependency < Requirement
   fatal true
+  default_formula "python"
 
   satisfy :build_env => false do
-    which_python
-  end
-
-  def which_python
-    @which_python ||= which python_binary
+    python = which_python
+    next unless python
+    version = python_short_version
+    next unless version
+    # Always use Python 2.7 for consistency on older versions of OSX.
+    version == Version.new("2.7")
   end
 
   def modify_build_environment
     if system_python?
-      if python_binary == 'python'
-        ENV['PYTHONPATH'] = "#{HOMEBREW_PREFIX}/lib/python2.7/site-packages"
+      if python_binary == "python"
+        version = python_short_version
+        ENV["PYTHONPATH"] = "#{HOMEBREW_PREFIX}/lib/python#{version}/site-packages"
       end
     elsif which_python
-      ENV.prepend_path 'PATH', which_python.dirname
+      ENV.prepend_path "PATH", which_python.dirname
     end
   end
 
-  def system_python?
-    which_python.to_s == "/usr/bin/#{python_binary}"
+  def python_short_version
+    @short_version ||= Language::Python.major_minor_version which_python
   end
 
-  def python_binary
-    'python'
+  def which_python
+    python = which python_binary
+    return unless python
+    executable = `#{python} -c "import sys; print(sys.executable)"`.strip
+    return unless executable
+    Pathname.new executable
   end
 
+  def system_python; "/usr/bin/#{python_binary}" end
+  def system_python?; system_python == which_python.to_s end
+  def python_binary; "python" end
+
   # Deprecated
   alias_method :to_s, :python_binary
 end
 
 class Python3Dependency < PythonDependency
-  default_formula 'python3'
+  fatal true
+  default_formula "python3"
 
-  satisfy :build_env => false do
-    which_python
-  end
+  satisfy(:build_env => false) { which_python }
 
-  def python_binary
-    'python3'
-  end
+  def python_binary; "python3" end
 end