Get rid of system variable where possible and use sys.platform or platform instead to allow type hinting

[C0rn3j]

Pyright is really unhappy with defining it into a var and does not eval the code paths properly as a result.

Right now it's just hardcoded to "Linux" and used even on Windows/macOS.

t_draw does not even use the code path with class RECT as a result.

[Taiko2k]

Originally on Windows, Tauon used the Win32 API for text rendering, I later migrated to using an msys2 environment and using Cairo for text as is done on Linux. Since in this environment many Linux code paths can remain the same and to simplify the code I now often use the "Linux" code paths. So I now have basically two modes on Windows, "Windows native", and MSYS. Largely system == "Windows", msys==false and system == "Linux", msys==true respectively.

The "Windows native" code path isn't currently used and hasn't been maintained in a long time and likely doesn't work as is, but I keep it around just in case its useful in the future. These blocks could be commented out.