Allow macosx thread safety test on macOS11

It appears to no longer timeout on that system, and is breaking CI,
which just switched to it.

Note, macOS plays games with the version number it reports depending on
how Python was compiled:
https://eclecticlight.co/2020/08/13/macos-version-numbering-isnt-so-simple/

Author: QuLogic

lib/matplotlib/tests/test_backends_interactive.py

@@ -222,8 +222,14 @@ def _test_thread_impl():
         param.marks.append(
             pytest.mark.xfail(raises=subprocess.CalledProcessError))
     elif backend == "macosx":
-        param.marks.append(
-            pytest.mark.xfail(raises=subprocess.TimeoutExpired, strict=True))
+        from packaging.version import parse
+        mac_ver = platform.mac_ver()[0]
+        # Note, macOS Big Sur is both 11 and 10.16, depending on SDK that
+        # Python was compiled against.
+        if mac_ver and parse(mac_ver) < parse('10.16'):
+            param.marks.append(
+                pytest.mark.xfail(raises=subprocess.TimeoutExpired,
+                                  strict=True))
     elif param.values[0].get("QT_API") == "PySide2":
         param.marks.append(
             pytest.mark.xfail(raises=subprocess.CalledProcessError))