Fix WebAgg after write_png changes
Merge pull request #3763 from mdboom/fix-webagg

BUG : fix type being passed to png writer in webAgg
MNT : delete unused import of Image

We only import Image to see if PIL/pillow is available to decide
if we should register jpg/tiff support for FigureCanvasBase.

Technically an API break, but anyone who was relying on this
probably deserves what they get.
Merge pull request #3765 from tacaswell/removed_unused_import

MNT : delete unused Image