zipfile: Fix regression that broke handling of zipfiles with internal…

… filenames not encoded in UTF-8. Fixes #1860889 [Decoding fails when opening a file with a specific file name in the viewer](https://bugs.launchpad.net/calibre/+bug/1860889)

src/calibre/utils/zipfile.py

@@ -11,7 +11,7 @@
 from calibre import sanitize_file_name
 from calibre.constants import filesystem_encoding
 from calibre.ebooks.chardet import detect
-from polyglot.builtins import unicode_type, string_or_bytes, getcwd, map
+from polyglot.builtins import unicode_type, string_or_bytes, getcwd, map, as_bytes
 
 try:
     import zlib  # We may need its compression method
@@ -327,8 +327,12 @@ def __init__(self, filename="NoName", date_time=(1980,1,1,0,0,0)):
         # This is used to ensure paths in generated ZIP files always use
         # forward slashes as the directory separator, as required by the
         # ZIP format specification.
-        if os.sep != "/" and os.sep in filename:
-            filename = filename.replace(os.sep, "/")
+        if os.sep != '/':
+            os_sep, sep = os.sep, '/'
+            if isinstance(filename, bytes):
+                os_sep, sep = as_bytes(os_sep), b'/'
+            if os_sep in filename:
+                filename = filename.replace(os_sep, sep)
 
         self.filename = filename        # Normalized file name
         self.date_time = date_time      # year, month, day, hour, min, sec