0.21

• 2D symbologies: PNG rendering is substantially faster, especially at large
  cell sizes, because upscaling is now done by Pillow rather than in Python.
• 2D symbologies: PNGs are now written at a bit depth of 1 by default, which
  gives noticeably smaller files. Black-and-white output is a 1-bit PNG; any
  other colours, including transparency, use a two-colour palette. Pass
  depth="full" to save, get_imagedata or png_dataurl to get
  the previous greyscale/colour output.
• 2D symbologies: get_pilimage has a new depth option. Its default,
  "full", returns the same image as before.
• 2D symbologies: save now always writes PNG, whatever the filename
  extension - this has always been the documented behaviour, but other
  formats may have worked.
• pyStrich is now tested on Python 3.10 to 3.15 (was Python 3.10 to 3.14).