fix: gray PNG loading with new version of imageio.

Loading a PNG image with imageio now returns a 2-channel image instead
of 4 channels with the first three repeated.

src/bob/io/base/__init__.py

@@ -104,11 +104,13 @@ def open_file(filename) -> np.ndarray:
 
         img = imageio.imread(filename)
 
-        # PNGs have a 4th channel, which we don't want
+        # PNGs have an additional channel, which we don't want
         # Alpha channels for instance have to be ignored
         if img.ndim > 2:
-            if extension.lower() == ".png":
-                img = img[:, :, 0:3]
+            if extension.lower() == ".png" and img.shape[-1] in (2, 4):
+                img = img[:, :, :-1]
+            if img.shape[-1] == 1:
+                img = img.squeeze(-1)
 
         img = check_gray(img)
         return img if img.ndim == 2 else to_bob(img)