class_docstr: Test remove digits.
class_docstr_tok: ["Test","remove","digits","."]
code: def test_remove_digits_phone(self):
      s = pd.Series("+41 1234 5678")
      s_true = pd.Series("+     ")
      self.assertEqual(preprocessing.remove_digits(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 6 for calling the TestPreprocessing obj's underlying member method code with example usage: obj.test_remove_digits_phone() without return types