The python formula explicity creates lib/python2.7/site-packages.
However, that path may already exist as a symlink if something that
installs python bindings is installed before python itself. The result
is that some random keg will end up hosting the site-packages directory
instead of the top-level directory.

Fix this by explicitly creating directories under lib/python* during
linking.

Fixes Homebrew/homebrew#11882.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>