Fix mutable flag transmission in .astype

As title. `np.ndarray.astype` always returns a mutable array.

Fixes numba#3844

numba/tests/test_array_methods.py

@@ -419,6 +419,11 @@ def check(arr, dtype):
         arr = np.arange(16, dtype=np.int32)[::2]
         check(arr, np.uint64)
 
+        # check read only attr does not get copied
+        arr = np.arange(16, dtype=np.int32)
+        arr.flags.writeable = False
+        check(arr, np.int32)
+
         # Invalid conversion
         dt = np.dtype([('x', np.int8)])
         with self.assertTypingError() as raises:

numba/typing/arraydecl.py

@@ -449,7 +449,9 @@ def resolve_astype(self, ary, args, kws):
                             "cannot convert from %s to %s"
                             % (dtype, ary, ary.dtype, dtype))
         layout = ary.layout if ary.layout in 'CF' else 'C'
-        retty = ary.copy(dtype=dtype, layout=layout)
+        # reset the write bit irrespective of whether the cast type is the same
+        # as the current dtype, this replicates numpy
+        retty = ary.copy(dtype=dtype, layout=layout, readonly=False)
         return signature(retty, *args)
 
     @bound_function("array.ravel")