python3: don't run ensurepip during build

We don't want to run ensurepip, which installs an old version of pip; we
take care of this ourselves in postinstall.

On flat (i.e. non-Framework) builds, ensurepip causes an error in postinstall
because the pip-3.4 script already exists in prefix/bin.

Will resolve Homebrew/linuxbrew#469.

Formula/python3.rb

@@ -101,6 +101,7 @@ class Python3 < Formula
       --datarootdir=#{share}
       --datadir=#{share}
       --enable-framework=#{frameworks}
+      --without-ensurepip
     ]
 
     args << "--without-gcc" if ENV.compiler == :clang