code: def test_notebook_contents_is_unicode(ipynb_file):
      nb = jupytext.read(ipynb_file)
  
      for cell in nb.cells:
          assert isinstance(cell.source, str)
  
file_path: jupytext/tests/functional/others/test_unicode.py
imports: from nbformat.v4.nbbase import new_markdown_cell, new_notebook
  import jupytext
  from jupytext.compare import compare
label_desc: Use image node_id 1 for calling a global function with example usage: test_notebook_contents_is_unicode(ipynb_file) without return types