Improve warning filtering in tests (#3175)

pyproject.toml

@@ -42,7 +42,10 @@ markers = [
 xfail_strict = true
 filterwarnings = [
     "error",
+    # this is mitigated by a try/catch in https://github.com/psf/black/pull/2974/
+    # this ignore can be removed when support for aiohttp 3.7 is dropped.
     '''ignore:Decorator `@unittest_run_loop` is no longer needed in aiohttp 3\.8\+:DeprecationWarning''',
+    # this is mitigated by https://github.com/python/cpython/issues/79071 in python 3.8+
+    # this ignore can be removed when support for 3.7 is dropped.
     '''ignore:Bare functions are deprecated, use async ones:DeprecationWarning''',
-    '''ignore:invalid escape sequence.*:DeprecationWarning''',
 ]

tests/test_format.py

@@ -28,6 +28,7 @@ def check_file(
     assert_format(source, expected, mode, fast=False)
 
 
+@pytest.mark.filterwarnings("ignore:invalid escape sequence.*:DeprecationWarning")
 @pytest.mark.parametrize("filename", all_data_cases("simple_cases"))
 def test_simple_format(filename: str) -> None:
     check_file("simple_cases", filename, DEFAULT_MODE)
@@ -132,6 +133,7 @@ def test_python_2_hint() -> None:
     exc_info.match(black.parsing.PY2_HINT)
 
 
+@pytest.mark.filterwarnings("ignore:invalid escape sequence.*:DeprecationWarning")
 def test_docstring_no_string_normalization() -> None:
     """Like test_docstring but with string normalization off."""
     source, expected = read_data("miscellaneous", "docstring_no_string_normalization")