Tests: Don't assume Python 3.10 is always installed, use current Python version

[hroncok]

See #3089 (review)

test_allowed_implicit_cli_envs[py310] assumed Python 3.10 is installed.
It failed without it:

____________________ test_allowed_implicit_cli_envs[py310] _____________________
[gw3] linux -- Python 3.12.0 /usr/bin/python3

env_name = 'py310'
tox_project = <function init_fixture.<locals>._init at 0x7febd4e1ca40>

    @pytest.mark.parametrize("env_name", ["py", "py310", ".pkg"])
    def test_allowed_implicit_cli_envs(env_name: str, tox_project: ToxProjectCreator) -> None:
        proj = tox_project({"tox.ini": ""})
        outcome = proj.run("r", "-e", env_name)
>       outcome.assert_success()

env_name   = 'py310'
outcome    = code: -1
cmd: /usr/bin/python3 -m tox r -e py310
cwd: /tmp/pytest-of-mockbuild/pytest-0/popen-gw3/test_allowed_implici...uld not find python interpreter with spec(s): py310
  py310: SKIP (0.23 seconds)
  evaluation failed :( (0.27 seconds)

proj       = ToxProject(path=/tmp/pytest-of-mockbuild/pytest-0/popen-gw3/test_allowed_implicit_cli_envs1/p) at 140650867026656
tox_project = <function init_fixture.<locals>._init at 0x7febd4e1ca40>

tests/session/test_env_select.py:158:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = code: -1
cmd: /usr/bin/python3 -m tox r -e py310
cwd: /tmp/pytest-of-mockbuild/pytest-0/popen-gw3/test_allowed_implici...uld not find python interpreter with spec(s): py310
  py310: SKIP (0.23 seconds)
  evaluation failed :( (0.27 seconds)

    def assert_success(self) -> None:
>       assert self.success, repr(self)  # noqa: S101
E       AssertionError: code: -1
E         cmd: /usr/bin/python3 -m tox r -e py310
E         cwd: /tmp/pytest-of-mockbuild/pytest-0/popen-gw3/test_allowed_implicit_cli_envs1/p
E         standard output
E         py310: skipped because could not find python interpreter with spec(s): py310
E           py310: SKIP (0.23 seconds)
E           evaluation failed :( (0.27 seconds)
E
E       assert False
E        +  where False = code: -1\ncmd: /usr/bin/python3 -m tox r -e py310\ncwd: /tmp/pytest-of-mockbuild/pytest-0/popen-gw3/test_allowed_implicit_cli_envs1/p\nstandard output\npy310: skipped because could not find python interpreter with spec(s): py310\n  py310: SKIP (0.23 seconds)\n  evaluation failed :( (0.27 seconds)\n.success

self       = code: -1
cmd: /usr/bin/python3 -m tox r -e py310
cwd: /tmp/pytest-of-mockbuild/pytest-0/popen-gw3/test_allowed_implici...uld not find python interpreter with spec(s): py310
  py310: SKIP (0.23 seconds)
  evaluation failed :( (0.27 seconds)

.../usr/lib/python3.12/site-packages/tox/pytest.py:352: AssertionError

This way, we use an environment that is available (as long as the tests run on CPython).
This could be improved to support PyPy as well, but I decided to take one step at a time.

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[hroncok]

Let me know if this needs a changelog fragment or not. I never know if it's required for test-only changes.

[hroncok]

Depends, if you want to release it soon, you need to add changelog otherwise not.

I don't need a release with this soon. I installed Python 3.10 for tests in the meantime.

[gaborbernat]