Pin python-docx version to temporarily fix upstream issues (#3612)

With the latest updates to python-docx, older APIs that are relied upon
by htmldocx have broken. This pins the version so htmldocx can utilize
the old APIs. For more info on this breakage see
[here](https://github.com/python-openxml/python-docx/issues/1256#issuecomment-1756839793).