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b4a6009c - rfctr(docx): improve typing etc. in prep for docx image extraction (#3015)

rfctr(docx): improve typing etc. in prep for docx image extraction (#3015) **Summary** Noisy but trivial changes to `partition_docx()` environs and tests in preparation for DOCX image extraction. These changes are extracted here so they don't distract on the changes of substance to follow in the next PR.