[1pt] Python 2 is gone, need to update page "Building from source"

[ligurio]

We are moving away from Python 2 to Python 3 (see tarantool/tarantool#5652),
let's update text and package names in https://www.tarantool.io/en/doc/latest/dev_guide/building_from_source/

[NickVolynkin]

@ligurio since which Tarantool versions is this change effective? Are they 2.5.3, 2.6.2, and 2.7.1?

[ligurio]

@NickVolynkin to be honest we are still in progress with migration (relevant tasks are [1] and [2]).
I believe change will affect all supported Tarantool versions.

1. Switch to python 3 by default in a test infrastructure tarantool#5652
2. Support Python 3 test-run#263

[ligurio]

We are close to moving Tarantool testing to Python 3:

• Switch to python 3 by default in a test infrastructure tarantool#5652 - to be merged next week
• Support Python 3 test-run#263

Change would affect every active release branch.

[Totktonada]

The transition to Python 3 is done.