In _dump(), use Python PPM save, instead of C

[radarhere]

from PIL import Image
im = Image.new("RGBA", (1, 1))
im._dump("out")

currently raises an error

Traceback (most recent call last):
  File "demo.py", line 3, in <module>
    im._dump("out")
  File "PIL/Image.py", line 749, in _dump
    self.im.save_ppm(filename)
ValueError: image has wrong mode

_dump uses the C level save_ppm() for this scenario, which doesn't support RGBA.

Pillow/src/PIL/Image.py

Lines 748 to 751 in b893310

	if not format or format == "PPM":
	self.im.save_ppm(filename)
	else:
	self.save(filename, format, **options)

Pillow/src/libImaging/File.c

Lines 60 to 68 in b893310

	if (im->mode == IMAGING_MODE_1 || im->mode == IMAGING_MODE_L) {
	/* Write "PGM" */
	fprintf(fp, "P5\n%d %d\n255\n", im->xsize, im->ysize);
	} else if (im->mode == IMAGING_MODE_RGB) {
	/* Write "PPM" */
	fprintf(fp, "P6\n%d %d\n255\n", im->xsize, im->ysize);
	} else {
	fclose(fp);
	(void)ImagingError_ModeError();

However, if we consistently use Python level save(), then it can be handled.

Pillow/src/PIL/PpmImagePlugin.py

Lines 333 to 341 in b893310

	def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
	if im.mode == "1":
	rawmode, head = "1;I", b"P4"
	elif im.mode == "L":
	rawmode, head = "L", b"P5"
	elif im.mode in ("I", "I;16"):
	rawmode, head = "I;16B", b"P5"
	elif im.mode in ("RGB", "RGBA"):
	rawmode, head = "RGB", b"P6"