Slightly simplify imgformat retrieval

No need for a loop here - we can access the enum like a dictionary,
which should be more efficient.

src/img2pdf.py

@@ -1832,11 +1832,9 @@ def read_images(
             )
     else:
         logger.debug("PIL format = %s", imgdata.format)
-        imgformat = None
+        try:
-        for f in ImageFormat:
+            imgformat = ImageFormat[imgdata.format]
-            if f.name == imgdata.format:
+        except KeyError:
-                imgformat = f
-        if imgformat is None:
             imgformat = ImageFormat.other
 
     def cleanup():