Review ignored warnings in selftests

filterwarnings = [
    "error",
    "default:Using or importing the ABCs:DeprecationWarning:unittest2.*",
    # produced by older pyparsing<=2.2.0.
    "default:Using or importing the ABCs:DeprecationWarning:pyparsing.*",
    "default:the imp module is deprecated in favour of importlib:DeprecationWarning:nose.*",
    # distutils is deprecated in 3.10, scheduled for removal in 3.12
    "ignore:The distutils package is deprecated:DeprecationWarning",
    # produced by python3.6/site.py itself (3.6.7 on Travis, could not trigger it with 3.6.8)."
    "ignore:.*U.*mode is deprecated:DeprecationWarning:(?!(pytest|_pytest))",
    # produced by pytest-xdist
    "ignore:.*type argument to addoption.*:DeprecationWarning",
    # produced on execnet (pytest-xdist)
    "ignore:.*inspect.getargspec.*deprecated, use inspect.signature.*:DeprecationWarning",
    # pytest's own futurewarnings
    "ignore::pytest.PytestExperimentalApiWarning",
    # Do not cause SyntaxError for invalid escape sequences in py37.
    # Those are caught/handled by pyupgrade, and not easy to filter with the
    # module being the filename (with .py removed).
    "default:invalid escape sequence:DeprecationWarning",
    # ignore use of unregistered marks, because we use many to test the implementation
    "ignore::_pytest.warning_types.PytestUnknownMarkWarning",
    # https://github.com/benjaminp/six/issues/341
    "ignore:_SixMetaPathImporter\\.exec_module\\(\\) not found; falling back to load_module\\(\\):ImportWarning",
    # https://github.com/benjaminp/six/pull/352 
    "ignore:_SixMetaPathImporter\\.find_spec\\(\\) not found; falling back to find_module\\(\\):ImportWarning",
    # https://github.com/pypa/setuptools/pull/2517
    "ignore:VendorImporter\\.find_spec\\(\\) not found; falling back to find_module\\(\\):ImportWarning",
]

That seems like a lot, and most of those don't actually have a filter for the module. It seems to me like this would potentially hide Python deprecation warnings in pytest itself. We should probably review those and either remove them, or add a module filter too.

Start with the TOML selftest warning-filter configuration shown in this issue and run the selftests to identify which warnings each entry suppresses. Review every broad filter, removing entries that are no longer needed or adding a module filter where appropriate; done means pytest warnings are not hidden unintentionally.