[seaborn] Use Literal for figure orientation arguments (#16716)

Author: srittau

stubs/seaborn/seaborn/_core/plot.pyi

@@ -115,7 +115,7 @@ class Plot:
         dpi: float | None = 96,
         facecolor: ColorType | Literal["auto"] | None = ...,
         edgecolor: ColorType | Literal["auto"] | None = ...,
-        orientation: str = "portrait",
+        orientation: Literal["landscape", "portrait"] = "portrait",
         format: str | None = None,
         bbox_inches: Literal["tight"] | Bbox | None = None,
         pad_inches: float | None = None,
@@ -140,7 +140,7 @@ class Plotter:
         dpi: float | None = 96,
         facecolor: ColorType | Literal["auto"] | None = ...,
         edgecolor: ColorType | Literal["auto"] | None = ...,
-        orientation: str = "portrait",
+        orientation: Literal["landscape", "portrait"] = "portrait",
         format: str | None = None,
         bbox_inches: Literal["tight"] | Bbox | None = None,
         pad_inches: float | None = None,