Python REPL doesn't launch using expected config on Ubuntu 20.04 LTS

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:

verify python is not installed, but python3 is
Opened a neoterm with :Ttoggle
Try to send line to terminal :TREPLSendLine
See E121: Undefined variable: g:neoterm_repl_command
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. ♥

@incoggnito can you help me with this one, please?

On Ubuntu, you can define the default version of Python that is being used, and the path for the binary, using update alternative. For instance, to set Python 3.8.5 as the default interpreter when typing python at the prompt, run (as root):

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.8.5 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.7.1 2

The number you put at the rightmost position defines the priority of a specific version of Python if you have multiple versions installed on your OS. You can query the defaults using sudo update-alternatives --config python. Usually, a single symlink /usr/bin/python3 -> /usr/bin/python should do the trick, though, or you can use some convenience package.

An alternative for @kassio would be to rely on python3_host_prog, instead of plain hard coded executable('python'), provided it is defined using let g:python3_host_prog = '/usr/bin/python3' in (neo)vim init file, as expected.

Hope that helps.

Still have this issue after a fresh install.

vim: neovim 0.6.1
OS: Windows

Opened a neoterm with :Ttoggle

Try to send line to terminal :TREPLSendLine

See E121: Undefined variable: g:neoterm_repl_command

See E116: Invalid arguments for function join(g:neoterm_repl_command) =~# 'ipython'

I'm using conda env, but the error exists even in base environment.