matplotlib: Use cbook.mkdirs instead of os.makedirs.
matplotlib: Remove catch for OSError.

os.access does not raise this exception.
matplotlib: _is_writable_dir checks that it is a directory.
matplotlib: _is_writable_dir tests with os.access and TemporaryFile.

This fails either if the OS tells us it isn't writable, or if a file cannot be
created there.