Make SkipTest.__init__ reason positional-only (#15703)

SkipTest does not override __init__ on any 3.x Python; it inherits
BaseException.__init__(self, *args), where *args is positional-only
at runtime. Without the / marker, the stub claims SkipTest(reason='x')
is valid, but it raises TypeError at runtime.

Author: adamtheturtle

stdlib/unittest/case.pyi

@@ -54,7 +54,7 @@ def skipIf(condition: object, reason: str) -> Callable[[_FT], _FT]: ...
 def skipUnless(condition: object, reason: str) -> Callable[[_FT], _FT]: ...
 
 class SkipTest(Exception):
-    def __init__(self, reason: str) -> None: ...
+    def __init__(self, reason: str, /) -> None: ...
 
 @type_check_only
 class _SupportsAbsAndDunderGE(SupportsDunderGE[Any], SupportsAbs[Any], Protocol): ...