Bytes and Big Integers

Python's PyCryptodome library implements this with the methods bytes_to_long() and long_to_bytes(). You will first have to install PyCryptodome and import it with from Crypto.Util.number import *. For more details check the FAQ.

Solution

from Crypto.Util.number import *

integer = 11515195063862318899931685488813747395775516287289682636499965282714637259206269
flag = long_to_bytes(integer).decode()
print(f"{flag}")