Implement /master/pgn/{game_id}

[Anupya]

Description

Implement get_otb_master_game = /master/pgn/{game_id}.

I did not add a return type for the function as there is no provided way to return ndjson/json in the response, but the response will be a PGN (text) due to the nature of the API endpoint.

After merge

Please check off /master/pgn/{game_id} in issue description of #6.

[benediktwerner]

The correct return type should be str in that case, together with format=PGN in the call.

[Anupya]

I don't think I need the format = PGN do I? since the response schema is application/x-chess-pgn.

[kraktus]

Thanks! As benediktwerner said, formatting as PGN is necessary otherwise it throws an exception when being called. I've included a test to confirm and fix that. Generally when you're not sure best to test!