pypy: unset PYTHONPATH while building

Fixes Homebrew/homebrew#24364.

Formula/pypy.rb

@@ -19,6 +19,11 @@ class Pypy < Formula
   end
 
   def install
+    # Having PYTHONPATH set can cause the build to fail if another
+    # Python is present, e.g. a Homebrew-provided Python 2.x
+    # See https://github.com/mxcl/homebrew/issues/24364
+    ENV['PYTHONPATH'] = ''
+
     rmtree 'site-packages'
 
     # The PyPy binary install instructions suggest installing somewhere