Eliminated the use of "bare" TypeVars in stdlib stubs (#5041)

Eliminated the use of "bare" TypeVars (i.e. a TypeVar that appears only once) within generic methods. While not considered an error in PEP 484, these are a common source of bugs in code, and some type checkers (including pytype and pyright) flag them as errors.

Co-authored-by: Eric Traut <erictr@microsoft.com>

stubs/click/click/termui.pyi

@@ -51,7 +51,7 @@ def progressbar(
     show_eta: bool = ...,
     show_percent: Optional[bool] = ...,
     show_pos: bool = ...,
-    item_show_func: Optional[Callable[[_T], str]] = ...,
+    item_show_func: Optional[Callable[[Any], str]] = ...,
     fill_char: str = ...,
     empty_char: str = ...,
     bar_template: str = ...,