[Python 3.12] Assertion rewriting triggers DeprecationWarning

What's the problem this feature will solve?

As of pytest 7.3.1, the classes ast.Str and ast.NameConstant are used in assertion rewriting. Their use has been deprecated in Python 3.8. These classes will emit a DeprecationWarning starting with Python 3.12.

When a user runs pytest 7.3.1 on Python 3.12and pytest collects a test with an assert statement, the test will show deprecation warnings.

Describe the solution you'd like

Pytest does not trigger a DeprecationWarning when running tests on Python 3.12.

Tested with pytest 7.3.1 on Python 3.12 (commit 0aeda29793):

test_anything.py

def test_a():
    assert True

$ pytest
===== test session starts =====
platform linux -- Python 3.12.0a7+, pytest-7.3.1, pluggy-1.0.0
rootdir: /tmp/test-cpython-next
collected 1 item                                                                                                                                                                                                                                                             

test_anything.py .                                                                                                                                                                                                                                                     [100%]

===== warnings summary =====
../venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:927
../venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:927
  /tmp/venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:927: DeprecationWarning: ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead
    assertmsg = ast.Str("")

../venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:929
  /tmp/venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:929: DeprecationWarning: ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead
    template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))

../venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:817
  /tmp/venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:817: DeprecationWarning: ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead
    keys = [ast.Str(key) for key in current.keys()]

../venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:941
../venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:941
  /tmp/venv-next/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:941: DeprecationWarning: ast.NameConstant is deprecated and will be removed in Python 3.14; use ast.Constant instead
    clear = ast.Assign(variables, ast.NameConstant(None))

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
===== 1 passed, 6 warnings in 0.01s ====