nvim-completion-manager: neovim shows a Python syntax error in cm.py when opening a new file, I despite no actual syntax error in file

In ~/.config/nvim/plugged/nvim-completion-manager/pythonx/cm.py, line 151 is

        self.nvim.call('cm#complete', name, ctx, startcol, matches, refresh, async=True)

I'm using Python 3.8 and copying the file into an online interpreter for Python does not result in any syntax errors, so it confuses me that neovim is interpreting it as such. I get Error detected while processing function <SNR>35_on_core_channel_error

My ~/.config/nvim/init.vim is

call plug#begin()
Plug 'roxma/nvim-completion-manager'
Plug 'SirVer/ultisnips'
Plug 'honza/vim-snippets'
call plug#end()

I tried replacing the leading spaces with tabs but I got the same output.

All of this is followed directly from https://www.linode.com/docs/guides/how-to-install-neovim-and-plugins-with-vim-plug/

File: https://onlinegdb.com/ByyfKgK6w (raises an error, but syntax is fine).

replace

Plug 'roxma/nvim-completion-manager'

with

Plug 'ncm2/ncm2'

in init.vim