0.5.0rc3

Candidate 3. rc2 had a bug around defining size as a class attribute. rc1 was missing the systems subpackage.

We went for a smaller release, but we got a lot done for it only being
a few months. The most important bits are that all of the input events
are in! Some cool stuff includes sprites scaling automatically and a
new way to move between scenes that uses the event system. That means
the old method is officially deprecated.

New stuff:

MouseButton events
Key events
Add a title to the game window
Sprite scaling based on game unit size
Keycodes flags
New scene change mechanism that uses the event system
Changed stuff:

Scene defaults are now class attributes
Most Sprite defaults are now class attributes
Flags can now be type hinted properly
Scenes no longer infinitely respawn their child scenes if running is
True.
Fixed an issue with the frame being different dimensions to the
viewport.
Fixed a bug in the Camera.point_in_viewport function
Default pixel ratio is now 64:1 (64 pixels to 1 game unit)
New (better) run function
Other type hinting fixes
Removed stuff:

bb attribute removed from sprites