Allow None in set_prop_cycle

`set_prop_cycle` allows to pass `None` for resetting the cycle, but with the current type hints, type checkers give an error for this case.

Author: vnmabus

lib/matplotlib/axes/_base.pyi

@@ -182,7 +182,7 @@ class _AxesBase(martist.Artist):
     def get_facecolor(self) -> ColorType: ...
     def set_facecolor(self, color: ColorType | None) -> None: ...
     @overload
-    def set_prop_cycle(self, cycler: Cycler) -> None: ...
+    def set_prop_cycle(self, cycler: Cycler | None) -> None: ...
     @overload
     def set_prop_cycle(self, label: str, values: Iterable[Any]) -> None: ...
     @overload