str.count only takes positional args (#11503)

```
>>> ''.count(x='a')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: str.count() takes no keyword arguments
```

Author: sobolevn

stdlib/builtins.pyi

@@ -447,7 +447,7 @@ class str(Sequence[str]):
     def center(self: LiteralString, __width: SupportsIndex, __fillchar: LiteralString = " ") -> LiteralString: ...
     @overload
     def center(self, __width: SupportsIndex, __fillchar: str = " ") -> str: ...  # type: ignore[misc]
-    def count(self, x: str, __start: SupportsIndex | None = ..., __end: SupportsIndex | None = ...) -> int: ...
+    def count(self, __sub: str, __start: SupportsIndex | None = ..., __end: SupportsIndex | None = ...) -> int: ...
     def encode(self, encoding: str = "utf-8", errors: str = "strict") -> bytes: ...
     def endswith(
         self, __suffix: str | tuple[str, ...], __start: SupportsIndex | None = ..., __end: SupportsIndex | None = ...