docstr: regression test for #1927
          
docstr_tok: ["regression","test","for","#","1927"]
code: def test_copy_categories_list(self):
      """regression test for #1927"""
      categories = ["a", "b", "c"]
  
      LabelArray(
          [None, "a", "b", "c"],
          missing_value=None,
          categories=categories,
      )
  
      # before #1927 we didn't take a copy and would insert the missing value
      # (None) into the list
      assert_equal(categories, ["a", "b", "c"])
  
imports: from itertools import product
  from operator import eq, ne
  import warnings
  import numpy
  from toolz import take
  from zipline.lib.labelarray import LabelArray
  from zipline.testing import check_arrays, parameter_space, ZiplineTestCase
  from zipline.testing.predicates import assert_equal
  from zipline.utils.compat import unicode
label_desc: Use image node_id 22 for calling the LabelArrayTestCase obj's underlying member method code with example usage: obj.test_copy_categories_list() without return types