Fix XRandr reporting displays on wayland session

XRandr doesn't support wayland. Using it doesn't do anything (no error, no brightness).

screen_brightness_control/linux.py

@@ -540,7 +540,7 @@ def _gdi(cls):
                     'manufacturer': None,
                     'manufacturer_id': None,
                     'edid': None,
-                    'unsupported': line.startswith('XWAYLAND')
+                    'unsupported': line.startswith('XWAYLAND') or 'WAYLAND_DISPLAY' in os.environ
                 }
                 display_count += 1