test: skip tkinter scheduler tests on PyPy (all platforms)

[dbrattli]

Summary

• The module-scope tkinter.Tk() in test_tkinterscheduler.py triggers a PyPy interpreter-shutdown crash: _tkinter/app.py __del__ → threading.notify_all → Fatal Python error: Aborted. Inside an xdist worker this kills the worker and fails whichever unrelated test it was running.
• The existing skip guard covered only macOS+PyPy. The same failure now surfaces on Linux+PyPy (observed on [Repo Assist] fix(operators): forward scheduler in pairwise, to_marbles, delay_with_mapper #757 CI, cascaded into cancelling the Windows and macOS PyPy jobs on the same run).
• Extend the skip guard to all PyPy platforms.

Example of the failure (from #757's run):

Fatal Python error: Aborted
  File \".../pypy3.10/_tkinter/app.py\", line 169 in __del__
  File \".../pypy3.10/threading.py\", line 389 in notify_all
  File \".../pypy3.10/threading.py\", line 568 in set
  File \".../xdist/remote.py\", line 100 in lock
  ...
[gw1] node down: Not properly terminated
FAILED tests/test_observable/test_case.py::TestCase::test_case_three

test_case_three has nothing to do with tkinter — it just happened to be on the worker when tkinter's finalizer aborted.

Test plan

• CI PyPy-3.10 (ubuntu) job passes without Fatal Python error: Aborted.
• CI PyPy-3.10 (macos) job still passes (same skip still applies).
• CI CPython jobs unchanged — pytest.importorskip(\"tkinter\") still runs on CPython, so tkinter tests execute where a display is available and skip otherwise.

🤖 Generated with Claude Code

[coveralls]

coverage: 93.45%. remained the same — fix/tkinter-test-skip-pypy-linux into master