Check for platform instead of system

[fohrloop]

The current implementation has three systems:

• windows
• linux
• darwin

These are all actually just directly platform.system().lower().

Might be better the change the name to be "platform" as python might be running on WSL or Cygwin on Windows..? Not yet sure if these two can be supported but at least is should be possible to detect that wakepy is running on WSL/Cygwin.

Task:

• Replace the SystemName with PlatformName class (or just platform module with constants WINDOWS, LINUX, and MACOS)
• Create function which can detect these platforms: Windows, Linux, MacOS