string.templatelib: do not allow concatenating Template with str (#14480)

Fixes #14479

Author: Dutcho

stdlib/string/templatelib.pyi

@@ -11,7 +11,7 @@ class Template:  # TODO: consider making `Template` generic on `TypeVarTuple`
 
     def __new__(cls, *args: str | Interpolation) -> Template: ...
     def __iter__(self) -> Iterator[str | Interpolation]: ...
-    def __add__(self, other: Template | str) -> Template: ...
+    def __add__(self, other: Template, /) -> Template: ...
     def __class_getitem__(cls, item: Any, /) -> GenericAlias: ...
     @property
     def values(self) -> tuple[Any, ...]: ...  # Tuple of interpolation values, which can have any type