Webkitpy's version check should not throw any exception.

The python version checker inside the webkitpy uses Python 2 syntax
to display the error message on the stderr. With Python 3 that code
throws an error. Using directly the stderr as a file object will
prevent the error.

BUG=408166
TEST=webkit-patch -h

Review URL: https://codereview.chromium.org/505353004

git-svn-id: svn://svn.chromium.org/blink/trunk@180994 bbb929c8-8fbe-4397-9dbb-9b2b20218538