python: Use PythonInstalled#binary

This simplyfies the formula a bit.
Also rearrange the line to be more logically grouped. Hopefully.

Library/Formula/python.rb

@@ -130,6 +130,8 @@ def install
     # We ship setuptools and pip and reuse the PythonInstalled
     # Requirement here to write the sitecustomize.py
     py = PythonInstalled.new("2.7")
+    py.binary = bin/'python'
+    py.modify_build_environment
 
     # Remove old setuptools installations that may still fly around and be
     # listed in the easy_install.pth. This can break setuptools build with
@@ -138,12 +140,10 @@ def install
     rm_rf Dir["#{py.global_site_packages}/setuptools*"]
     rm_rf Dir["#{py.global_site_packages}/distribute*"]
 
-    py.binary = bin/'python'
-    py.modify_build_environment
     setup_args = [ "-s", "setup.py", "--no-user-cfg", "install", "--force", "--verbose",
                    "--install-scripts=#{bin}", "--install-lib=#{site_packages}" ]
-    Setuptools.new.brew { system "#{bin}/python2", *setup_args }
-    Pip.new.brew { system "#{bin}/python2", *setup_args }
+    Setuptools.new.brew { system py.binary, *setup_args }
+    Pip.new.brew { system py.binary, *setup_args }
 
     # And now we write the distuitsl.cfg
     cfg = prefix/"Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/distutils.cfg"