Ensure keyword list only contains string

This commit ensures that every keyword in the keyword list that is to be written
to a sidecar or exif only contains strings. The current implementation fails
with an exception as the "sorted" functions will fail if the list contains a mix
of strings and integers.

osxphotos/photoinfo/_photoinfo_export.py

@@ -1443,7 +1443,7 @@ def _exiftool_dict(
 
     if keyword_list:
         # remove duplicates
-        keyword_list = sorted(list(set(keyword_list)))
+        keyword_list = sorted(list(set([str(keyword) for keyword in keyword_list])))
         exif["IPTC:Keywords"] = keyword_list.copy()
         exif["XMP:Subject"] = keyword_list.copy()
         exif["XMP:TagsList"] = keyword_list.copy()