test: suppress an ast deprecation warning

For example:
```
.tox/anypy/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:1004: 50 warnings
/Users/nedbatchelder/coverage/trunk/.tox/anypy/lib/python3.12/site-packages/_pytest/assertion/rewrite.py:1004: DeprecationWarning: ast.Str is deprecated a  nd will be removed in Python 3.14; use ast.Constant instead
    expl_format = self.pop_format_context(ast.Str(expl))
```

pyproject.toml

@@ -40,7 +40,6 @@ filterwarnings = [
     "ignore:the imp module is deprecated in favour of importlib:DeprecationWarning",
     "ignore:distutils Version classes are deprecated:DeprecationWarning",
     "ignore:The distutils package is deprecated and slated for removal in Python 3.12:DeprecationWarning",
-    "ignore:is deprecated and will be removed in Python 3.14; use ast.Constant instead:DeprecationWarning",
 ]
 
 # xfail tests that pass should fail the test suite

tests/conftest.py

@@ -54,13 +54,6 @@ def set_warnings() -> None:
         message=r"module 'sre_constants' is deprecated",
     )
 
-    # From _pytest/assertion/rewrite.py
-    warnings.filterwarnings(
-        "ignore",
-        category=DeprecationWarning,
-        message=r"is deprecated and will be removed in Python 3.14; use ast.Constant instead",
-    )
-
     warnings.filterwarnings(
         "ignore",
         category=pytest.PytestRemovedIn8Warning,

tox.ini

@@ -30,6 +30,7 @@ setenv =
     # For some tests, we need .pyc files written in the current directory,
     # so override any local setting.
     PYTHONPYCACHEPREFIX=
+    PYTHONWARNINGS=ignore:removed in Python 3.14; use ast.Constant:DeprecationWarning
 
 # $set_env.py: COVERAGE_PIP_ARGS - Extra arguments for `pip install`
 # `--no-build-isolation` will let tox work with no network.