Add thread_unsafe to __all__ and test.rst

Author: colesbury

Doc/library/test.rst

@@ -792,6 +792,11 @@ The :mod:`test.support` module defines the following functions:
    Decorator for invoking :func:`check_impl_detail` on *guards*.  If that
    returns ``False``, then uses *msg* as the reason for skipping the test.
 
+.. decorator:: thread_unsafe(reason=None)
+
+   Decorator for marking tests as thread-unsafe.  This test always runs in one
+   thread even when invoked with ``--parallel-threads``.
+
 
 .. decorator:: no_tracing
 

Lib/test/support/__init__.py

@@ -40,7 +40,7 @@
     "anticipate_failure", "load_package_tests", "detect_api_mismatch",
     "check__all__", "skip_if_buggy_ucrt_strfptime",
     "check_disallow_instantiation", "check_sanitizer", "skip_if_sanitizer",
-    "requires_limited_api", "requires_specialization",
+    "requires_limited_api", "requires_specialization", "thread_unsafe",
     # sys
     "MS_WINDOWS", "is_jython", "is_android", "is_emscripten", "is_wasi",
     "is_apple_mobile", "check_impl_detail", "unix_shell", "setswitchinterval",