I was able to hunt down the issue.

The problem is in python-nav-end-of-block where there is an implicit
assumption that python-nav-end-of-statement always makes forward
progress, which is violated when the buffer contains e.g. a single
quote.

I have attached a patch the resolves this issue but there may be other
lurking issues due to the removal of cl-assert because there are now
all sorts of code paths that will run that never ran before because
they would hit that assertion and bail out, usually all the way back
to the top.