Implemented logic to mutate array's writable flag

Example:

```
import dpctl.tensor as dpt, pytest

x = dpt.ones((2,2))
x.flags.writable = False
with pytest.raises(ValueError):
   x[:] = 0  # can not mutate read-only array
x.flags["W"] = True
x[:] = 0   # works fine now
```

Author: oleksandr-pavlyk

dpctl/tensor/_flags.pyx

@@ -75,6 +75,12 @@ cdef class Flags:
         """
         return _check_bit(self.flags_, USM_ARRAY_WRITABLE)
 
+    @writable.setter
+    def writable(self, new_val):
+        if not isinstance(new_val, bool):
+            raise TypeError("Expecting a boolean value")
+        self.arr_._set_writable_flag(new_val)
+
     @property
     def fc(self):
         """
@@ -129,6 +135,14 @@ cdef class Flags:
         elif name == "CONTIGUOUS":
             return self.forc
 
+    def __setitem__(self, name, val):
+        if name in ["WRITABLE", "W"]:
+            self.writable = val
+        else:
+            raise ValueError(
+                "Only writable ('W' or 'WRITABLE') flag can be set"
+            )
+
     def __repr__(self):
         out = []
         for name in "C_CONTIGUOUS", "F_CONTIGUOUS", "WRITABLE":

dpctl/tensor/_usmarray.pxd

@@ -72,4 +72,6 @@ cdef api class usm_ndarray [object PyUSMArrayObject, type PyUSMArrayType]:
     cdef dpctl.DPCTLSyclQueueRef get_queue_ref(self) except *
     cdef dpctl.SyclQueue get_sycl_queue(self)
 
+    cdef _set_writable_flag(self, int)
+
     cdef __cythonbufferdefaults__ = {"mode": "strided"}

dpctl/tensor/_usmarray.pyx

@@ -532,6 +532,12 @@ cdef class usm_ndarray:
         """
         return _flags.Flags(self, self.flags_)
 
+    cdef _set_writable_flag(self, int flag):
+        cdef int arr_fl = self.flags_
+        arr_fl ^= (arr_fl & USM_ARRAY_WRITABLE)  # unset WRITABLE flag
+        arr_fl |= (USM_ARRAY_WRITABLE if flag else 0)
+        self.flags_ = arr_fl
+
     @property
     def usm_type(self):
         """
@@ -1390,12 +1396,10 @@ cdef api Py_ssize_t UsmNDArray_GetOffset(usm_ndarray arr):
     allocation"""
     return arr.get_offset()
 
+
 cdef api void UsmNDArray_SetWritableFlag(usm_ndarray arr, int flag):
     """Set/unset USM_ARRAY_WRITABLE in the given array `arr`."""
-    cdef int arr_fl = arr.flags_
-    arr_fl ^= (arr_fl & USM_ARRAY_WRITABLE)  # unset WRITABLE flag
-    arr_fl |= (USM_ARRAY_WRITABLE if flag else 0)
-    arr.flags_ = arr_fl
+    arr._set_writable_flag(flag)
 
 cdef api object UsmNDArray_MakeSimpleFromMemory(
     int nd, const Py_ssize_t *shape, int typenum,