PythonInstalled: Unset PYTHONPATH for satisfied?

When a formula `depends_on :python` *and* `depends_on :python3`
the `modify_build_environment` method sets the PYTHONPATH
and the Python 3.x requirement then fails because it finds
the sitecustomize.py from Python 2.x in the PYTHONPATH.

Library/Homebrew/requirements/python_dependency.rb

@@ -74,6 +74,7 @@ def initialize(default_version="2.6", tags=[])
   # We look for a brewed python or an external Python and store the loc of
   # that binary for later usage. (See Formula#python)
   satisfy :build_env => false do
+    ENV['PYTHONPATH'] = nil
     @unsatisfied_because = ''
     if binary.nil? || !binary.executable?
       @unsatisfied_because += "No `#{@name}` found in your PATH! Consider to `brew install #{@name}`."