Describe the bug
Ubuntu doesn't ship with python anymore, but ships with python3 so the check is_valid which evaluates executable('python') should possibly include executable('python3') or something.... (I think the way is_valid is handled is not very clear, although maybe it's the simplest way).

• is_valid:

  neoterm/ftdetect/set_repl_cmd.vim

  Line 13 in 8a3a0fb

  \ if neoterm#repl#python#is_valid(g:neoterm_repl_python) == 1 |

• executable:

  neoterm/autoload/neoterm/repl/python.vim

  Line 13 in 018f662

  return executable('python')

To Reproduce
Steps to reproduce the behavior:

1. verify python is not installed, but python3 is
2. Opened a neoterm with :Ttoggle
3. Try to send line to terminal :TREPLSendLine
4. See E121: Undefined variable: g:neoterm_repl_command
5. See E116: Invalid arguments for function join(g:neoterm_repl_command) =~# 'ipython'

Expected behavior
Configuration according to https://github.com/kassio/neoterm/blob/master/doc/neoterm.txt#L272 should allow me to execute code in a REPL.

Screenshots/gifs
N/A

Versions (Issues without this information will take longer to be answered/solved):

• OS: Ubuntu 20.04.1 LTS
• neoterm commit master/807a94f746bac2ab328dc22e6ca88bd411283cc8
• Add all the configuration you have for neoterm

let g:neoterm_repl_python =
  \ ['source .virtualenv/bin/activate', 'python']

" This makes it "work":
let g:neoterm_repl_command = add(g:neoterm_repl_python, '')

• Vim or Neovim
  • vim
  • neovim
• Version [vim --version] VIM - Vi IMproved 8.1 (2018 May 18, compiled Apr 15 2020 06:40:31)

Additional context
Hope I provided enough context. Awesome plugin. Thank you for your hard work. ♥