Load the source for the specified file.

def load_source(self):
    """Load the source for the specified file."""
    if self.filename in self.STDIN_NAMES:
      self.filename = 'stdin'
      self.source = pycodestyle.stdin_get_value()
    else:
      with pep257.tokenize_open(self.filename) as fd:
        self.source = fd.read()