I am trying out your patch at the moment. There are plenty of test suite failures; I ran the test suite with approximately the following:

./python -bWerror -m test -Wr -j0 -u network -x test_{mailbox,shelve,faulthandler,multiprocessing_main_handling,venv,warnings}

Importing modules sometimes fails or generates the warning, but this goes away if the file is not out of date. E.g. run “touch Lib/test/test_codecs.py”, and then make sure you next import that module with -Wall or -Werror enabled.

374 tests OK.
10 tests failed:
    test___all__ test_ast test_codecs test_doctest test_fstring
    test_idle test_strlit test_trace test_unicode
    test_zipimport_support

I started pasting some of the failures here, but gave up as more and more failed. Let me know if you want the full details.

======================================================================
ERROR: test_coverage (test.test_trace.TestCoverage)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/test/test_trace.py", line 312, in test_coverage
    self._coverage(tracer)
  File "/media/disk/home/proj/python/cpython/Lib/test/test_trace.py", line 307, in _coverage
    r.write_results(show_missing=True, summary=True, coverdir=TESTFN)
  File "/media/disk/home/proj/python/cpython/Lib/trace.py", line 284, in write_results
    lnotab = _find_executable_linenos(filename)
  File "/media/disk/home/proj/python/cpython/Lib/trace.py", line 403, in _find_executable_linenos
    code = compile(prog, filename, "exec")
DeprecationWarning: invalid escape sequence 'w'
**********************************************************************
File "/media/disk/home/proj/python/cpython/Lib/test/test_doctest.py", line 288, in test.test_doctest.test_DocTest
Failed example:
    docstring = '''
        >>> print(12)
        12

    Non-example text.

        >>> print('another\example')
        another
        example
    '''
Exception raised:
    Traceback (most recent call last):
      File "/media/disk/home/proj/python/cpython/Lib/doctest.py", line 1330, in __run
        compileflags, 1), test.globs)
    DeprecationWarning: invalid escape sequence 'e'
**********************************************************************
[Many subsequent NameError exceptions from test_doctest]
**********************************************************************
File "/tmp/tmphzbypj98/test_zip.zip/test_zipped_doctest.py", line 288, in test_zipped_doctest.test_DocTest
Failed example:
    docstring = '''
        >>> print(12)
        12

    Non-example text.

        >>> print('another\example')
        another
        example
    '''
Exception raised:
    Traceback (most recent call last):
      File "/media/disk/home/proj/python/cpython/Lib/doctest.py", line 1330, in __run
        compileflags, 1), test.globs)
    DeprecationWarning: invalid escape sequence 'e'
**********************************************************************
[More failures]

======================================================================
FAIL: test_all (test.test___all__.AllTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/test/test___all__.py", line 105, in test_all
    self.check_all(modname)
  File "/media/disk/home/proj/python/cpython/Lib/test/test___all__.py", line 28, in check_all
    raise FailedImport(modname)
  File "/media/disk/home/proj/python/cpython/Lib/contextlib.py", line 89, in __exit__
    next(self.gen)
  File "/media/disk/home/proj/python/cpython/Lib/test/support/__init__.py", line 1130, in _filterwarnings
    raise AssertionError("unhandled warning %s" % reraise[0])
AssertionError: unhandled warning {message : DeprecationWarning("invalid escape sequence '('",), category : 'DeprecationWarning', filename : '/media/disk/home/proj/python/cpython/Lib/importlib/_bootstrap.py', lineno : 222, line : None}

======================================================================
ERROR: test_escape_order (test.test_fstring.TestCase) (str='f\'{"a"\\!r}\'')
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/test/test_fstring.py", line 20, in assertAllRaise
    eval(str)
DeprecationWarning: invalid escape sequence '!'

======================================================================
ERROR: test_escape (test.test_codecs.EscapeDecodeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/test/test_codecs.py", line 1218, in test_escape
    decode(b"\\" + b)
OverflowError: character argument not in range(0x110000)

======================================================================
ERROR: test_escape_decode (test.test_codecs.UnicodeEscapeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/test/test_codecs.py", line 2467, in test_escape_decode
    check(br"[\8]", r"[\8]")
  File "/media/disk/home/proj/python/cpython/Lib/test/test_codecs.py", line 26, in check
    self.assertEqual(coder(input), (expect, len(input)))
DeprecationWarning: invalid escape sequence '8'

test test_unicode crashed -- Traceback (most recent call last):
  File "/media/disk/home/proj/python/cpython/Lib/test/libregrtest/runtest.py", line 167, in runtest_inner
    the_module = importlib.import_module(abstest)
  File "/media/disk/home/proj/python/cpython/Lib/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 996, in _gcd_import
  File "<frozen importlib._bootstrap>", line 979, in _find_and_load
  File "<frozen importlib._bootstrap>", line 968, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 663, in exec_module
  File "<frozen importlib._bootstrap_external>", line 770, in get_code
  File "<frozen importlib._bootstrap_external>", line 730, in source_to_code
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
DeprecationWarning: invalid escape sequence '?'