Fix a type hint issue

Author: seisman

pygmt/clib/conversion.py

@@ -201,7 +201,7 @@ def _to_numpy(data: Any) -> np.ndarray:
         numpy_dtype = getattr(dtype, "numpy_dtype", None)
         if Version(pd.__version__) < Version("2.1"):
             # In pandas 2.0, dtype.numpy_type is dtype("O").
-            numpy_dtype = np.dtype(f"M8[{dtype.pyarrow_dtype.unit}]")
+            numpy_dtype = np.dtype(f"M8[{dtype.pyarrow_dtype.unit}]")  # type: ignore[assignment]
 
     array = np.ascontiguousarray(data, dtype=numpy_dtype)