CHM Input: Fix a regression in calibre 5.0 that broke opening of some files that dont specify a character encoding. Fixes #1922503 [E-book viewer: error opening a *.chm file](https://bugs.launchpad.net/calibre/+bug/1922503)

py3 compat

src/calibre/ebooks/chm/reader.py

@@ -77,6 +77,8 @@ def get_encoding(self):
                 ans = lcid[0]
         if ans:
             try:
+                if isinstance(ans, bytes):
+                    ans = ans.decode('ascii')
                 codecs.lookup(ans)
             except Exception:
                 ans = None