FYI, I found a way to build the library for python3 in Ubuntu 22.04:
wget https://github.com/NationalAssociationOfRealtors/libRETS/archive/1.6.6.tar.gz
tar xzf 1.6.6.tar.gz
cd libRETS-1.6.6
libtoolize --force
aclocal
autoheader
automake --force-missing --add-missing
autoconf
autoupdate
./autogen.sh
./configure --disable-perl --disable-ruby --disable-java --disable-php --disable-node.js --disable-sql-compiler
make
FYI, I found a way to build the library for python3 in Ubuntu 22.04: