Skip writable_dir test on Windows
Skip writable_dir test on Windows

It doesn't seem possible to create a dir that the owner can't write to
on Windows, so the test doesn't run properly.  If the test can be
updated to reliably create/find a user read-only directory (probably via
NTFS ACLs and win32security), then it can be unskipped.

ipython/ipython,ipython/ipython