Fix python3.13 deprecation warning

[sdb9696]

Hi, we've been updating our CI to work with python3.13 in preparation for the upcoming release in October. While doing so we received DeprecationWarnings from checker.py passing count to re.sub as a positional argument. It seems in python3.13 this is deprecated due to confusion between flags and count.

This PR updates checker.py to pass re.MULTILINE to re.sub with a keyword flags argument. I think the original code intended for this to be a flag rather than a count. When I tested this with re.MULTILINE passed as the 4th argument it interpreted it as an integer of 8 and only replaces 8 occurrences of BLANKLINE_MAKER.

>>> from xdoctest.checker import remove_blankline_marker
>>> BLANKLINE_MARKER = '<BLANKLINE>'
>>> BLANKLINE_MARKER_ARR = [BLANKLINE_MARKER] * 10
>>> text='Foo\n'.join(BLANKLINE_MARKER_ARR )
>>> remove_blankline_marker(text)
'\nFoo\nFoo\nFoo\nFoo\nFoo\nFoo\nFoo\nFoo\n<BLANKLINE>Foo\n<BLANKLINE>'

Running the test suite against main on python 3.13:

tests/test_checker.py::test_visible_lines
tests/test_checker.py::test_blankline_accept
tests/test_checker.py::test_blankline_failcase
tests/test_doctest_example.py::test_failed_assign_want
tests/test_doctest_example.py::test_failed_assign_want
tests/test_doctest_example.py::test_eval_expr_capture
tests/test_traceback.py::test_lineno_failcase_gotwant
tests/test_traceback.py::test_lineno_failcase_gotwant
  ~/xdoctest/src/xdoctest/checker.py:637: DeprecationWarning: 'count' is passed as positional argument
    new_text = re.sub(blankline_pattern, '\n', text, re.MULTILINE)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
== 296 passed, 23 skipped, 8 warnings in 43.77s ==

Running the test suite against this PR on python 3.13:

== 296 passed, 23 skipped in 44.35s ==

[Erotemic]

Thanks for catching this. Can you add a "Fixed" entry to CHANGELOG.md?

I will merge and push a release this fix shortly.

[sdb9696]

Thanks for catching this. Can you add a "Fixed" entry to CHANGELOG.md?

I will merge and push a release this fix shortly.

That's done