Fix type hint for hexbin(reduce_C_function=...)

We only ever pass a list internally.

Author: QuLogic

lib/matplotlib/axes/_axes.pyi

@@ -433,7 +433,7 @@ class Axes(_AxesBase):
         alpha: float | None = ...,
         linewidths: float | None = ...,
         edgecolors: Literal["face", "none"] | ColorType = ...,
-        reduce_C_function: Callable[[np.ndarray], float] = ...,
+        reduce_C_function: Callable[[np.ndarray | list[float]], float] = ...,
         mincnt: int | None = ...,
         marginals: bool = ...,
         *,

lib/matplotlib/pyplot.py

@@ -3101,7 +3101,7 @@ def hexbin(
     alpha: float | None = None,
     linewidths: float | None = None,
     edgecolors: Literal["face", "none"] | ColorType = "face",
-    reduce_C_function: Callable[[np.ndarray], float] = np.mean,
+    reduce_C_function: Callable[[np.ndarray | list[float]], float] = np.mean,
     mincnt: int | None = None,
     marginals: bool = False,
     *,