unittest: make protocol parameter positional-only (#12834)

Author: JelleZijlstra

stdlib/unittest/runner.pyi

@@ -13,13 +13,14 @@ class _SupportsWriteAndFlush(SupportsWrite[str], SupportsFlush, Protocol): ...
 
 # All methods used by unittest.runner.TextTestResult's stream
 class _TextTestStream(_SupportsWriteAndFlush, Protocol):
-    def writeln(self, arg: str | None = None) -> str: ...
+    def writeln(self, arg: str | None = None, /) -> str: ...
 
 # _WritelnDecorator should have all the same attrs as its stream param.
 # But that's not feasible to do Generically
 # We can expand the attributes if requested
 class _WritelnDecorator(_TextTestStream):
     def __init__(self, stream: _TextTestStream) -> None: ...
+    def writeln(self, arg: str | None = None) -> str: ...
     def __getattr__(self, attr: str) -> Any: ...  # Any attribute from the stream type passed to __init__
     # These attributes are prevented by __getattr__
     stream: Never