Replace PIL.Image.DEBUG checks by logging at DEBUG level?

[anntzer]

All's basically in the title. I don't have a strong argument except that it looks nicer, imo.

[homm]

+1 Pillow should use standard logging.

[aclark4life]

Anyone want to estimate LOE required?

[anntzer]

#1207 already covers most of what's needed.

[aclark4life]

Can you fix up the PR? I'll merge this right after 2.9.0

[radarhere]

Since the PR has been merged, this issue is complete.