It's held up on SSL. Ubuntu 20.04 changed some security parameter tunings, which breaks some uses of the SSL module, and approximately eight modules in the test suite. I assume this wasn't caught on the buildbots because they don't use Ubuntu--or at least not a build that fresh. SSL and the test suite are all completely happy on older Ubuntu releases.
One could argue "it's fine, most people still using 3.5 are also
using old OSes anyway". But I don't want to release 3.5.10 if
important functionality is broken on a popular OS. So I'm waiting
for help from the ssl module maintainer(s) who are very kindly
looking into it.
My plan is to release 3.5.10rc1 once it passes the test suite on Ubuntu 20.04... whenever that is. 3.5.10 final will be automatically rescheduled for two weeks from that date.
p.s. Happy American Independence Day!