Skip Tkinter tests with threading on PyPy.

It does not support calls to Tkinter from different threads:
https://foss.heptapod.net/pypy/pypy/-/issues/1929

Author: QuLogic

lib/matplotlib/tests/test_backend_tk.py

@@ -1,6 +1,7 @@
 import functools
 import inspect
 import os
+import platform
 import re
 import subprocess
 import sys
@@ -111,6 +112,9 @@ def legitimate_quit():
         print("success")
 
 
+@pytest.mark.skipif(platform.python_implementation() != 'CPython',
+                    reason='PyPy does not support Tkinter threading: '
+                           'https://foss.heptapod.net/pypy/pypy/-/issues/1929')
 @pytest.mark.backend('TkAgg', skip_on_importerror=True)
 @pytest.mark.flaky(reruns=3)
 @_isolated_tk_test(success_count=1)

lib/matplotlib/tests/test_backends_interactive.py

@@ -3,6 +3,7 @@
 import inspect
 import json
 import os
+import platform
 import signal
 import subprocess
 import sys
@@ -220,6 +221,12 @@ def _test_thread_impl():
     elif param.values[0].get("QT_API") == "PySide2":
         param.marks.append(
             pytest.mark.xfail(raises=subprocess.CalledProcessError))
+    elif backend == "tkagg" and platform.python_implementation() != 'CPython':
+        param.marks.append(
+            pytest.mark.xfail(
+                reason='PyPy does not support Tkinter threading: '
+                       'https://foss.heptapod.net/pypy/pypy/-/issues/1929',
+                strict=True))
 
 
 @pytest.mark.parametrize("env", _thread_safe_backends)