virtual environment indicator removed if only python3 installed

[GregDMeyer]

Describe the bug

If there is no command python present on the system, but Python 3 is installed (e.g. by default on recent Ubuntu distros), the virtual environment item is removed from the prompt.

Steps to reproduce

1. Run a machine that has only python3 but not python available
2. Generate a virtual environment with python3 -m venv test_env
3. Run source test_env/bin/activate.fish
4. Virtual environment indicator is not present on the right hand side of prompt

Environment

Output of tide bug-report:

fish version: 3.2.2
tide version: 4.3.4
term: xterm-256color
os: Pop OS 20.10
terminal emulator: Gnome terminal

Additional context

I did a tiny bit of digging and it looks like the function _tide_remove_unusable_items removes the virtual_env item if the command python is not present (it does not check if python3 etc. may be available instead).

[IlanCosman]

Nice catch!