BUG: fix __setitem__ for invalid dtype combinations

Do not allow e.g.

>>> a = xp.asarray([1, 2, 3])
>>> a[0] = 3.5     # cannot type-promote a float scalar and an int array
>>> a[0] = xp.asarray(3.5)   # cannot integer and float arrays cannot be promoted together

Author: ev-br

array_api_strict/_array_object.py

@@ -942,6 +942,15 @@ def __setitem__(
         self._validate_index(key, op="setitem")
         # Indexing self._array with array_api_strict arrays can be erroneous
         np_key = key._array if isinstance(key, Array) else key
+
+        # sanitize the value
+        other = value
+        if isinstance(value, (bool, int, float, complex)):
+            other = self._promote_scalar(value)
+        dt = _result_type(self.dtype, other.dtype)
+        if dt != self.dtype:
+            raise ValueError(f"mismatched dtypes: {self.dtype = } and {other.dtype = }")
+
         self._array.__setitem__(np_key, asarray(value)._array)
 
     def __sub__(self, other: Array | complex, /) -> Array: