hi _az,
I removed the errant library via: apt-get remove libpython3.5-minimal (which was where that 3.5.4-2 came from), then I was able to install as normal. Problem solved.
FYI. to anyone else looking at this thread. The solution (and _az's suggestions) demonstrated that the problem was an erroneously installed (mis-matching versions) python3 library package, and had nothing to do with certbot at all. certbot was just the unlucky end program that exposed the problem.
Many thanks for your clarity in helping me find the solution!
R.