class_docstr: Test remove digits.
class_docstr_tok: ["Test","remove","digits","."]
code: def test_replace_digits(self):
      s = pd.Series("1234 falcon9")
      s_true = pd.Series("X falcon9")
      self.assertEqual(preprocessing.replace_digits(s, "X"), 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 8 for calling the TestPreprocessing obj's underlying member method code with example usage: obj.test_replace_digits() without return types