Revert "Use de-dupe idiom that works with Python 3.6, refs #1632"

This reverts commit 5bfd001b55.

No need for this on the main branch because it doesn't support Python 3.6 any more.

datasette/cli.py

@@ -550,9 +550,7 @@ def serve(
                 )
 
     # De-duplicate files so 'datasette db.db db.db' only attaches one /db
-    files_seen = set()
-    deduped_files = [f for f in files if f not in files_seen and not files_seen.add(f)]
-    files = deduped_files
+    files = list(dict.fromkeys(files))
 
     try:
         ds = Datasette(files, **kwargs)