[3.13] gh-143346: Fix calculation of the line width for wrapped Base64 in plistlib (GH-143347) (GH-143720)

It was incorrect in case of mixed tabs and spaces in indentation.
(cherry picked from commit 5f28aa2f372339ba0c70373b96d33ec4d2879e04)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>