add screen size macros

[zenodante]

Hi, I added SCREEN_WIDTH and SCREEN_HEIGHT defines to the picosystem.hpp. Then the code in picosystem.cpp could be simplified a little.

[Gadgetoid]

sigh the Python linting fails are my bad, I didn't pin the flake8 version so it's picking up things that previously didn't register as an error.

With that said, uh, do you mind... fixing them... and popping on another commit so I can merge this? 😆

test.py's update should be:

def update(tick):
    global view

    if pressed(RIGHT):
        view += 1

    if pressed(LEFT):
        view -= 1

    view %= view_count

[Gadgetoid]

The above fixes have been added in #87 - you should just be able to rebase this PR and it'll pass CI!