Revert default python to python3

[artem-shelkovnikov]

Follow up for #2734 (review):

This was not a backwards-compatible change, and it did not work well for me when I was testing - python alias always led to system Python.

But it looks, like it just should. If CI is green, then I was wrong and we can safely keep the old python3 command work - python3 should be correctly handled by pyenv

Some context on python vs python3: https://stackoverflow.com/questions/67550189/i-dont-have-python-but-i-have-python3

Recent Ubuntu versions do not install python 2 by default, as python 2 is now "dead". However, the command python is not (yet) linked to Python 3, to avoid some confusion; traditionally, python was for python 2 while python3 was for python 3.

[github-actions]

💔 Failed to create backport PR(s)

The backport operation could not be completed due to the following error:
There are no branches to backport to. Aborting.

The backport PRs will be merged automatically after passing CI.

To backport manually run:
backport --pr 2748 --autoMerge --autoMergeMethod squash