python_dependency: cleanup and fix build env.

* Only set PYTHONPATH for Python 2.
* Set the Python binary for superenv.

References Homebrew/homebrew#24842.
Closes Homebrew/homebrew#26197.
Closes Homebrew/homebrew#26216.
Closes Homebrew/homebrew#26218.
Closes Homebrew/homebrew#26228.

Library/Homebrew/requirements/python_dependency.rb

@@ -4,23 +4,43 @@ class PythonDependency < Requirement
   fatal true
 
   satisfy :build_env => false do
-    which 'python'
+    which_python
+  end
+
+  def which_python
+    @which_python ||= which python_binary
   end
 
   def modify_build_environment
-    ENV['PYTHONPATH'] = "#{HOMEBREW_PREFIX}/lib/python2.7/site-packages"
+    if python_binary == 'python'
+      ENV['PYTHONPATH'] = "#{HOMEBREW_PREFIX}/lib/python2.7/site-packages"
+    end
+
+    if !system_python? && which_python
+      ENV.prepend_path 'PATH', which_python.dirname
+    end
+  end
+
+  def system_python?
+    which_python.to_s == "/usr/bin/#{python_binary}"
+  end
+
+  def python_binary
+    'python'
   end
 
   # Deprecated
-  def to_s
-    'python'
-  end
+  alias_method :to_s, :python_binary
 end
 
 class Python3Dependency < PythonDependency
   default_formula 'python3'
 
   satisfy :build_env => false do
-    which 'python3'
+    which_python
+  end
+
+  def python_binary
+    'python3'
   end
 end