I found "libboost_python35.so.1.75.0" by grep -a on forexconnect/lib/libboost_python.so.1.64.0.
This file is installed with forexconnect so forms part of the package.
There is no reference in the .whl file modified to work with python3.8, so heaven knows why that link is there at all.
Looking in the python3.7 version, what looks like the equivalent reference to libboost_python3.so.1.64.0 in the file of the same name.
Copying libboost_python.so.1.64.0 to libboost_python35.so.1.75.0 appears to work but that is not an ideal solution! But at least I can use python3.8 now as required.