title: numba jitclass - attribute a.x is a.x returns False
body: <p>Trying to check the identity of two jitclass instances attributes, I found this strange behaviour compared to ordinary python classes. Numba jitclass return False on its own instance attribute identity check where ordinary python classes work as expected.</p>
  
  <pre><code>import numba
  
  class MyClass(object):
      def __init__(self, x):
          self.x = x
  
  spec = [('x', numba.double)]
  @numba.jitclass(spec)
  class MyJitClass(object):
      def __init__(self, x):
          self.x = x
  
  a = MyClass(1)
  b = MyJitClass(1)
  </code></pre>
  
  <p>Now checking the identity:</p>
  
  <pre><code>&gt;&gt;&gt; a.x is a.x
  True
  &gt;&gt;&gt; b.x is b.x
  False
  </code></pre>
  
  <p>Any idea why this happens? And how I should check whether two jitclass attributes reference to the same object?</p>