PEP 553 built-in breakpoint() function (bpo-31353) (#3355)

Implement PEP 553, built-in breakpoint() with support from sys.breakpointhook(), along with documentation and tests.  Closes bpo-31353
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 7cc1e78..819fcc5 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -3523,7 +3523,8 @@
         needs_semantic_update = {"round"}
         no_signature |= needs_semantic_update
         # These need *args support in Argument Clinic
-        needs_varargs = {"min", "max", "print", "__build_class__"}
+        needs_varargs = {"breakpoint", "min", "max", "print",
+                         "__build_class__"}
         no_signature |= needs_varargs
         # These simply weren't covered in the initial AC conversion
         # for builtin callables