Supporting maim for screenshots on Linux

[gacou54]

Related to #28

Since Fedora Linux distribution does not support scrot anymore (https://bugzilla.redhat.com/show_bug.cgi?id=1770415), I add the support for maim (highly inspired by https://github.com/asweigart/pyautogui/pull/83/files#).

The tests pass and the pyscreeze library (and pyautogui) now works on my Fedora machine.

[socal-nerdtastic]

Why not simply

mainExists = not subprocess.call(['which', 'maim'])

[gacou54]

I simply used the same code as for the scrot detection in order to conform to the current style of the code.

If the maintainer doesn't mind refactoring some code (the scrot detection), subprocresse.call would be better in my opinion, but otherwise, I think it's better to stick to the current code style.

[Avasam]

@gacou54 This now has conflicts

[gacou54]

The idea to add maim was to make pyscreeze usable for wayland systems.

Adding gnome-screenshot fixes this. You can close this PR, unless you really want to support maim.