py-xlsxwriter: updated to 3.1.5

Release 3.1.5 - September 24 2023
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* Added support for adding signed VBA macros to workbooks via the via the
  :func:`add_signed_vba_project` method. See `Working with VBA Macros`_.

Release 3.1.4 - September 18 2023
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* Added support for enabling the Excel "Show #N/A as an empty cell" chart option
  via the :func:`show_na_as_empty_cell` method.

(adam)