Log Message:
py-xlsxwriter: updated to 1.3.7

Release 1.3.7

* Fixed issue where custom chart data labels didn't inherit the position of
  the data labels in the series.

* Added text alignment for textboxes. The existing options allowed the text
  area to be aligned but didn't offer control over the text within that area.

* Added Python 3.9 to the test matrix.