tests: skip Tk tests on macOS only

Author: BoboTiG

mss/tests/test_issue_220.py

@@ -10,8 +10,8 @@
 
 tkinter = pytest.importorskip("tkinter")
 
-if platform.python_implementation() == "PyPy":
-    # PyPy 7.3.11 [Python 3.9.16] fails on GitHub:
+if platform.system().lower() == "darwin" and platform.python_implementation() == "PyPy":
+    # [macOS] PyPy 7.3.11 [Python 3.9.16] fails on GitHub:
     #     RuntimeError: tk.h version (8.5) doesn't match libtk.a version (8.6)
     pytestmark = pytest.mark.skip