update FLIP_LEFT_RIGHT definition to fix deprecation warning

[richardmonette]

I noticed this warning which was occurring when using the script.

/Users/richardmonette/src/linedraw/linedraw.py:97: DeprecationWarning: FLIP_LEFT_RIGHT is deprecated and will be removed in Pillow 10 (2023-07-01). Use Transpose.FLIP_LEFT_RIGHT instead.
  IM2 = IM.rotate(-90,expand=True).transpose(Image.FLIP_LEFT_RIGHT)

Went ahead and made a quick PR to fix 🙂