Add pypy support about pyimgui

[stuaxo]

The bug has been fixed in nightly pypy and pyimgui works with the cocos2d backend.

I wasn't able to get the pygame or pysdl2 backends to work, and wasn't sure what to install to try glfw3.

[swistakm]

Great! Let's leave this issue open until there will be some official release of pypy that we can safely include in the test pipeline.

[stuaxo]

I'm not sure when they do releases, but the biggest gap seems to be about every 6 months, so we should check this by 6 months at the latest.

I need to try this with pypy3 nightly too in the meantime.

[stuaxo]

The pygame example seems to work with pygame-cffi, pygame itself doesn't quite install in pypy yet.