yt_data_extract: import append_po_token

Import the append_po_token function so the function is accessible from
watch page.

Author: alive4ever

youtube/yt_data_extract/__init__.py

@@ -9,5 +9,5 @@
 from .watch_extraction import (extract_watch_info, get_caption_url,
     update_with_new_urls, requires_decryption,
     extract_decryption_function, decrypt_signatures, _formats, replace_n_signatures,
-    update_format_with_type_info, extract_hls_formats,
+    update_format_with_type_info, extract_hls_formats, append_po_token,
     extract_watch_info_from_html, captions_available)