cbz2ebook: Use mt map for opening image and zip

This apparently solves some weird issues with reading the zip file like

zlib.error: Error -3 while decompressing data: invalid block type
zlib.error: Error -3 while decompressing data: invalid distance too far back

Though it reduced the amount of times this happened, some files still
persistently refuse to be read by zipfile in parallel. In those cases,
using `-s` to force single-threading should be sufficient.

cbz2ebook.py

@@ -70,7 +70,7 @@ else:
 
 for idx, im in enumerate(
     tqdm(
-        mapper(process_image, map(Image.open, map(srczip.open, srcfiles)),),
+        mapper(process_image, mapper(Image.open, mapper(srczip.open, srcfiles)),),
         total=len(srcfiles),
     )
 ):