Exception occurred:
  File "/home/vstinner/prog/python/master/Doc/venv/lib/python3.9/site-packages/docutils/io.py", line 245, in __init__
    self.source = open(source_path, mode, **kwargs)
ValueError: invalid mode: 'rU'

class FileInput(Input):
    def __init__(self, source=None, source_path=None,
                 encoding=None, error_handler='strict',
                 autoclose=True, mode='rU', **kwargs):
        ...
        if source is None:
            if source_path:
                ...
                try:
                    self.source = open(source_path, mode, **kwargs)
                except IOError as error:
                    raise InputError(error.errno, error.strerror, source_path)