linux: add PyGObject as a dependency

This dependency is required for proper support of the AppIndicator
backend from pystray:

https://github.com/moses-palmer/pystray/blob/v0.19.4/docs/faq.rst#how-do-i-use-pystray-in-a-virtualenv-on-linux

requirements.txt

@@ -2,6 +2,7 @@ aiohttp>2.0,<4.0
 Pillow
 # TODO: switch back to a stable pystray release when a version newer than v0.19.4 is out
 pystray@git+https://github.com/moses-palmer/pystray.git@903836104f8b1a2979f4fa5a333870ef7b075992
+PyGObject; sys_platform == "linux" # required for better system tray support on Linux
 # selenium-wire
 # undetected-chromedriver
 # this is installed only on windows