src/img2pdf.py: use logging.warning because warn() is deprecated

src/img2pdf.py

@@ -638,10 +638,10 @@ def get_imgmetadata(imgdata, imgformat, default_dpi, colorspace, rawdata=None):
         ics = imgdata.mode
 
     if ics in ["LA", "PA", "RGBA"]:
-        logging.warn("Image contains transparency which cannot be retained in PDF.")
-        logging.warn("img2pdf will not perform a lossy operation.")
-        logging.warn("You can remove the alpha channel using imagemagick:")
-        logging.warn("  $ convert input.png -background white -alpha remove -alpha off output.png")
+        logging.warning("Image contains transparency which cannot be retained in PDF.")
+        logging.warning("img2pdf will not perform a lossy operation.")
+        logging.warning("You can remove the alpha channel using imagemagick:")
+        logging.warning("  $ convert input.png -background white -alpha remove -alpha off output.png")
         raise Exception("Refusing to work on images with alpha channel")