linux: pin PyGObject version to <3.51

PyGObject 3.51 (and newer) now requires girepository-2.0 as a build
dependency [1], which is a version we cannot easily install in the
Ubuntu version we use in CI/CD, so the build fails:

=======

$ python3 -m pip install -r requirements.txt
[ ... ]
Collecting PyGObject (from -r requirements.txt (line 4))
  Downloading pygobject-3.52.3.tar.gz (1.2 MB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 1.2/1.2 MB 45.4 MB/s eta 0:00:00
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
  Preparing metadata (pyproject.toml): started
  Preparing metadata (pyproject.toml): finished with status 'error'
  error: subprocess-exited-with-error

  × Preparing metadata (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [216 lines of output]
      + meson setup /tmp/pip-install-nayte7bo/pygobject_097c5326983b4ca690e238e49ba4b29c /tmp/pip-install-nayte7bo/pygobject_097c5326983b4ca690e238e49ba4b29c/.mesonpy-sp71efmg -Dbuildtype=release -Db_ndebug=if-release -Db_vscrt=md -Dtests=false -Dwheel=true --wrap-mode=nofallback --native-file=/tmp/pip-install-nayte7bo/pygobject_097c5326983b4ca690e238e49ba4b29c/.mesonpy-sp71efmg/meson-python-native-file.ini
      The Meson build system
      Version: 1.7.0
      Source dir: /tmp/pip-install-nayte7bo/pygobject_097c5326983b4ca690e238e49ba4b29c
      Build dir: /tmp/pip-install-nayte7bo/pygobject_097c5326983b4ca690e238e49ba4b29c/.mesonpy-sp71efmg
      Build type: native build
      Project name: pygobject
      Project version: 3.52.3
      C compiler for the host machine: cc (gcc 9.4.0 "cc (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0")
      C linker for the host machine: cc ld.bfd 2.34
      Host machine cpu family: x86_64
      Host machine cpu: x86_64
      Program python3 found: YES (/opt/hostedtoolcache/Python/3.10.16/x64/bin/python3)
      Found pkg-config: YES (/usr/bin/pkg-config) 1.6.3
      Run-time dependency python found: YES 3.10
      Found CMake: /usr/local/bin/cmake (3.31.6)
      Run-time dependency girepository-2.0 found: NO (tried pkgconfig and cmake)
      Not looking for a fallback subproject for the dependency girepository-2.0 because:
      Use of fallback dependencies is disabled.

      ../meson.build:31:9: ERROR: Dependency 'girepository-2.0' is required but not found.

=======

Unfortunately, girepository-2.0 is only available in Ubuntu 24.10, a
version we can't easily use in GitHub Actions. But even if could, doing
so would also needlessly bump the minimum glibc version requirement for
Linux users, making TDM much less portable.

So, for now, let's pin PyGObject to <3.51.

[1]: d29f4e65fa/NEWS (L4-5)

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<3.51; sys_platform == "linux"  # required for better system tray support on Linux
 
 # environment-dependent dependencies
 pywin32; sys_platform == "win32"