Log Message:
py-openpyxl: update to 2.6.4

This is a release with a single bug fix that corrects a file corruption
issue.

Cannot save workbooks with comments more than once.
https://foss.heptapod.net/openpyxl/openpyxl/-/issues/1330

(There is a newer 3.x branch with many more changes since, but it drops
Python 2.7 support and is more involved, so can't be committed during
the pkgsrc freeze.)