convert eps using pnmraw instead of ppmraw

This lets Ghostscript choose the best device to use (pbmraw, pgmraw, ppmraw) based on the image data.

Tests/test_file_eps.py

@@ -178,7 +178,7 @@ def test_load_long_binary_data(prefix: bytes) -> None:
     data = io.BytesIO(prefix + b"\n".join(simple_eps_file_with_long_binary_data))
     with Image.open(data) as img:
         img.load()
-        assert img.mode == "RGB"
+        assert img.mode == "1"
         assert img.size == (100, 100)
         assert img.format == "EPS"
 

src/PIL/EpsImagePlugin.py

@@ -121,7 +121,13 @@ def Ghostscript(
                 lengthfile -= len(s)
                 f.write(s)
 
-    device = "pngalpha" if transparency else "ppmraw"
+    if transparency:
+        # "RGBA"
+        device = "pngalpha"
+    else:
+        # "pnmraw" automatically chooses between
+        # PBM ("1"), PGM ("L"), and PPM ("RGB").
+        device = "pnmraw"
 
     # Build Ghostscript command
     command = [