Remove call to deprecated Element.getchildren.

Fixes ocrmypdf#583.

src/ocrmypdf/hocrtransform.py

@@ -110,7 +110,7 @@ def _get_element_text(self, element):
  text = ''
  if element.text is not None:
  text += element.text
- for child in element.getchildren():
+ for child in element:
  text += self._get_element_text(child)
  if element.tail is not None:
  text += element.tail