Install instructions are here: Salt Stack on Ubuntu.
So in my version of salt, v2017.7.4 (Nitrogen), I was getting this error when I tried to use the file_tree ext_pillar:
Failed to load ext_pillar file_tree: must be str, not bytes
So I monkey patched my version of /usr/lib/python3/dist-packages/salt/pillar/file_tree.py, changing the file from binary to text:
contents = '' try: # 2020-05-15 jj5 - changed 'rb' to 'r', will only work with text files... with salt.utils.fopen(file_path, 'r') as fhr: buf = fhr.read(__opts__['file_buffer_size']) while buf: contents += buf buf = fhr.read(__opts__['file_buffer_size']) if contents.endswith('\n') \ and _check_newline(prefix, file_name, keep_newline): contents = contents[:-1]