python: don't run ensurepip during build

This is the default setting for python 2.7, but we're applying this
change to python3, so we may as well apply the symmetric change here in
case the default changes in future micro releases.

Closes Homebrew/homebrew#41544.

Signed-off-by: Tim D. Smith <git@tim-smith.us>

Formula/python.rb

@@ -95,6 +95,7 @@ class Python < Formula
       --datarootdir=#{share}
       --datadir=#{share}
       --enable-framework=#{frameworks}
+      --without-ensurepip
     ]
 
     args << "--without-gcc" if ENV.compiler == :clang