Changes in 0.17.3:
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* Added initial work on a Mallard xml2po mode (Shaun McCance)
* Internationalization work on Mallard stylesheets (Milo Casagrande)
* Fixed example in Mallard document (Baptiste Mille-Mathias)
* Preserve exiting PYTHONPATH in bootstrap (David Adam)
* Addressed RTL issues in Mallard stylesheets (Shaun McCance)
* Updated translations:
  ar      (Khaled Hosny)
  de      (Mario Blättermann)
  et      (Ivar Smolin)
  pt_BR   (Leonardo Ferreira Fontenelle)
  sv      (Daniel Nylander)