title: Counter in range() not recognized as integer
body: <p>I am trying to step through a list of lists operating on each element of each list using nested for loops.  I am getting a warning from PyCharm that the type of the counter in the second for loop is not certain to be an integer, despite it is derived from a range value. The code executes correctly, by why the warning?</p>
  
  <pre><code>def get_vote_fraction(cl_count, ag_vector):
      v_f_vector = [[0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0, 0]]
      for b in range(0, len(v_f_vector)):
          for c in range(0, len(v_f_vector[b])):
              v_f_vector[b][c] = f"{(ag_vector[b][c] / cl_count): .2F}"
      return v_f_vector
  
  
  aggregated_vector = [[0, 8, 0, 6], [0, 1, 0, 0, 0, 0, 9, 0], [0, 0, 10, 0], [0, 10, 0, 0, 0]]
  class_count = 10
  vote_fraction = get_vote_fraction(class_count, aggregated_vector)
  print(vote_fraction)
  </code></pre>
  
  <p>As expected the output is [[' 0.00', ' 0.80', ' 0.00', ' 0.60'], [' 0.00', ' 0.10', ' 0.00', ' 0.00', ' 0.00', ' 0.00', ' 0.90', ' 0.00'], [' 0.00', ' 0.00', ' 1.00', ' 0.00'], [' 0.00', ' 1.00', ' 0.00', ' 0.00', ' 0.00']]</p>
  
  <p>I get a warning that c in v_f_vector[b][c] can be of Unexpected type(s) (int, str) which says c is not sufficiently defined as an integer.  I hesitate to raise an issue with PyCharm since I may be missing something simple. Anyone see what I am missing?</p>