This is not the problem of hls4ml nor pynq. It is python & pip configuration problem.
One of the easiest and reliable solution to the python problems is installing anaconda ecosystem, Anaconda | Anaconda Distribution
Install it, load basic envvars (source PATH_TO_YOUR_CONDA_PATH/etc/profile.d/conda.sh) then activate environment (conda activate default)
or you can setup your individual setups.
Probably you have to install pip in the conda (conda install pip)
Then proceed to the steps to install hls4ml (git clone, pip install .)
Well, I guess this is already solved issue, simply not updated 
and expect to hear some news that you were able to build a transformer python <-> hdl 