add small note about pi

in module 2 the pi character is used. there should be an explanation that python recognizes unicode characters, and that that can be copy/pasted or directly specified with a unicode code. nerds can learn more [here](https://docs.python.org/3/howto/unicode.html#unicode-literals-in-python-source-code) or [here](https://realpython.com/python-encodings-guide/)

print('π')
print('\u03C0')