recommend to use python3 when python is not found

We can easily show a message like this:
The program 'python' can be found in the following packages:
 * python3
 * python-minimal
Try: sudo apt-get install <selected pacakge>

However there is currently no mechanism to suggest a installed alternative. This is trivial to add but it would require a string addition.

This bug was fixed in the package command-not-found - 0.3ubuntu13

---------------
command-not-found (0.3ubuntu13) utopic; urgency=low

  * add python3 suggestion if a user types "python"
    (LP: #1306682)
  * do not crash for invalid unicode (LP: #1130444)
  * update scan.data to latest utopic
 -- Michael Vogt <email address hidden> Wed, 16 Jul 2014 08:03:59 +0200

reopening ...

$ python
The program 'python' can be found in the following packages:
 * python-minimal
 * python3
Try: sudo apt-get install <selected package>

python3 should be suggested as the priority
python3-minimal should be suggested instead of python-minimal

and I'm not sure if the hint is sufficient. I would have expected something along the lines:
"you typed python, did you mean python3, which is already installed on the system?"

This bug was fixed in the package command-not-found - 20.04.0

---------------
command-not-found (20.04.0) focal; urgency=medium

  * Special case python -> python3, using existing translated strings. LP:
    #1306682, LP: #1863532

 -- Dimitri John Ledkov <email address hidden> Thu, 12 Mar 2020 13:10:32 +0000