Set encoding to utf-8 when saving json file for stickerpack

Fixes UnicodeEncodeError with Windows 10 when trying to import sticker packs caused by the default encoding scheme in Windows

sticker/stickerimport.py

@@ -99,7 +99,7 @@ async def reupload_pack(client: TelegramClient, pack: StickerSetFull, output_dir
                 doc["body"] = sticker.emoticon
             doc["net.maunium.telegram.sticker"]["emoticons"].append(sticker.emoticon)
 
-    with open(pack_path, "w") as pack_file:
+    with open(pack_path, "w", encoding="utf-8") as pack_file:
         json.dump({
             "title": pack.set.title,
             "id": f"tg-{pack.set.id}",