Make the first argument of float.__new__ positional-only (#8991)

Closes #8990

Author: LeeeeT

stdlib/builtins.pyi

@@ -299,7 +299,7 @@ class int:
     def __index__(self) -> int: ...
 
 class float:
-    def __new__(cls: type[Self], x: SupportsFloat | SupportsIndex | str | ReadableBuffer = ...) -> Self: ...
+    def __new__(cls: type[Self], __x: SupportsFloat | SupportsIndex | str | ReadableBuffer = ...) -> Self: ...
     def as_integer_ratio(self) -> tuple[int, int]: ...
     def hex(self) -> str: ...
     def is_integer(self) -> bool: ...