TYP: npyio: loadtxt: usecols: add None type

According to the previous type hint, None was not allowed, but since None is the default value, then None IS allowed

Author: jlecordier

numpy/lib/_npyio_impl.pyi

@@ -158,7 +158,7 @@ def loadtxt(
     delimiter: None | str = ...,
     converters: None | Mapping[int | str, Callable[[str], Any]] = ...,
     skiprows: int = ...,
-    usecols: int | Sequence[int] = ...,
+    usecols: int | Sequence[int] | None = ...,
     unpack: bool = ...,
     ndmin: L[0, 1, 2] = ...,
     encoding: None | str = ...,
@@ -175,7 +175,7 @@ def loadtxt(
     delimiter: None | str = ...,
     converters: None | Mapping[int | str, Callable[[str], Any]] = ...,
     skiprows: int = ...,
-    usecols: int | Sequence[int] = ...,
+    usecols: int | Sequence[int] | None = ...,
     unpack: bool = ...,
     ndmin: L[0, 1, 2] = ...,
     encoding: None | str = ...,
@@ -192,7 +192,7 @@ def loadtxt(
     delimiter: None | str = ...,
     converters: None | Mapping[int | str, Callable[[str], Any]] = ...,
     skiprows: int = ...,
-    usecols: int | Sequence[int] = ...,
+    usecols: int | Sequence[int] | None = ...,
     unpack: bool = ...,
     ndmin: L[0, 1, 2] = ...,
     encoding: None | str = ...,