linux: pin PyGObject version to <3.47

This fixes the broken "Linux (AppImage)" CI job.

PyGObject 3.47 has dropped setuptools in favor of meson-python, which
isn't a problem per se, but they also require meson 0.63.3, which we
don't currently have in the repositories of Ubuntu 20.04, which is
the oldest (yet supported) version we use to build our AppImage
package, in order to increase compatibility with as much Linux distros
as possible.

And since there's no real, compelling reason to upgrade to the latest
version of PyGObject right now, let's just pin it to version <3.46,
until we eventually upgrade the Ubuntu version we use for CI.

requirements.txt

@@ -1,7 +1,7 @@
 aiohttp>=3.9,<4.0
 Pillow
 pystray
-PyGObject; sys_platform == "linux" # required for better system tray support on Linux
+PyGObject<3.47; sys_platform == "linux" # required for better system tray support on Linux
 
 # environment-dependent dependencies
 pywin32; sys_platform == "win32"