The problem is that tcl/tk 8.5 and lower does not support png icons only gif ones when used in this way. That said, tcl/tk 8.6 and later support png for icons in this way.
The problem is that Python 3.5 building from source on Mac OS X seems to now be hard coded to link to tck/tk 8.5 when previously they linked to either 8.5 or 8.6.
See
https://www.python.org/download/mac/tcltk/
So the default build will no longer dynamically link to tck/tk 8.6 which has the fix.
This is so sad! I am going to have to rebuild the bundled Python and somehow path the build to actually look for and require 8.6 and not the stupid 8.5.
I will look to see what changed from building Python 3.4 to 3.5 and see if I can back out that change.
What a mess!
And why farting with this I have broken my portable Python 3.5 build completely so no more Sigil builds until I get it fixed!
KevinH