class_docstr: Test remove digits.
class_docstr_tok: ["Test","remove","digits","."]
code: def test_tokenize(self):
      s = pd.Series("text to tokenize")
      s_true = pd.Series([["text", "to", "tokenize"]])
      self.assertEqual(preprocessing.tokenize(s), s_true)
  
imports: import string
  import pytest
  import pandas
  import numpy
  import doctest
  from texthero import preprocessing, stopwords
  from .None import PandasTestCase
  import warnings
label_desc: Use image node_id 18 for calling the TestPreprocessing obj's underlying member method code with example usage: obj.test_tokenize() without return types