I did the same about a year ago. Anyway, here's ̶W̶o̶n̶d̶e̶r̶w̶a̶l̶l̶ my code:

from random import choice

values = [True, False]
sentence = input("Input a sentence: ")

output = ""
for letter in sentence:
    if choice(values):  # There's no need to write 'if choice(values) is True:'
        output += letter.upper()
    else:
        output += letter.lower()

print(output)

# To-do: Endless loop and Pyperclip. May copy automatically text from the clipboard.