sys: add last_exc on Python>=3.12 (#11378)

https://docs.python.org/3/library/sys.html#sys.last_exc

stdlib/sys/__init__.pyi

@@ -42,6 +42,8 @@ hexversion: int
 last_type: type[BaseException] | None
 last_value: BaseException | None
 last_traceback: TracebackType | None
+if sys.version_info >= (3, 12):
+    last_exc: BaseException  # or undefined.
 maxsize: int
 maxunicode: int
 meta_path: list[_MetaPathFinder]

tests/stubtest_allowlists/py312.txt

@@ -76,6 +76,9 @@ types.GenericAlias.__mro_entries__
 sys._monitoring  # Doesn't really exist. See comments in the stub.
 weakref.ProxyType.__reversed__  # Doesn't really exist
 
+# sys attributes that are not always defined
+sys.last_exc
+
 # C signature is broader than what is actually accepted
 ast.ExtSlice.__new__
 ast.Index.__new__