CI - fix flaky type check failure in cirq.vis.heatmap (#6750)

Avoid `Incompatible types in assignment` error by providing explicit
cast of the value returned from `get_facecolor()`.

The failure is flaky and more likely to happen if run without mypy cache
This fix was verified using:

    for i in {1..50}; do check/mypy --cache-dir=/dev/null || break; done

Author: pavoljuhas

cirq-core/cirq/vis/heatmap.py

@@ -28,7 +28,6 @@
     SupportsFloat,
     Tuple,
     Union,
-    TYPE_CHECKING,
 )
 
 import matplotlib as mpl
@@ -40,8 +39,6 @@
 from cirq.devices import grid_qubit
 from cirq.vis import vis_utils
 
-if TYPE_CHECKING:
-    from numpy.typing import ArrayLike
 
 QubitTuple = Tuple[grid_qubit.GridQubit, ...]
 
@@ -240,8 +237,8 @@ def _write_annotations(
         ax: plt.Axes,
     ) -> None:
         """Writes annotations to the center of cells. Internal."""
-        facecolor: ArrayLike
-        for (center, annotation), facecolor in zip(centers_and_annot, collection.get_facecolor()):
+        face_colors = cast(np.ndarray, collection.get_facecolor())
+        for (center, annotation), facecolor in zip(centers_and_annot, face_colors):
             # Calculate the center of the cell, assuming that it is a square
             # centered at (x=col, y=row).
             if not annotation: