class_docstr: Test remove digits.
class_docstr_tok: ["Test","remove","digits","."]
code: def test_pipeline_stopwords(self):
      s = pd.Series("E-I-E-I-O\nAnd on")
      s_true = pd.Series("e-i-e-i-o\n ")
      pipeline = [
          preprocessing.lowercase,
          preprocessing.remove_stopwords,
      ]
      self.assertEqual(
          preprocessing.clean(s, pipeline=pipeline), 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 13 for calling the TestPreprocessing obj's underlying member method code with example usage: obj.test_pipeline_stopwords() without return types