title: scipy.sparse lil_matrix: __iadd__ operator
body: <p>I am facing a very strange behaviour in the computation of the sum bewteen two lil_matrix. Please find below the code</p>
  <pre><code>from scipy.sparse import lil_matrix
  a = lil_matrix([100])
  print(a.todense())
  a += lil_matrix([-100])
  print(a.todense())
  </code></pre>
  <p>I would expect a result equals to 0 as it happens with</p>
  <pre><code>print((lil_matrix([100])+lil_matrix([-100])).todense())
  </code></pre>
  <p>Can someone help understanding the issue?</p>
  <p>please find below the versions
  python 3.8.5
  scipy 1.5.2</p>
  <p>Thanks</p>