import argparse
import xml.sax.handler
from typing import Optional
from typing import Sequence


def main(argv: Optional[Sequence[str]] = None) -> int:
    parser = argparse.ArgumentParser()
    parser.add_argument('filenames', nargs='*', help='XML filenames to check.')
    args = parser.parse_args(argv)

    retval = 0
    handler = xml.sax.handler.ContentHandler()
    for filename in args.filenames:
        try:
            with open(filename, 'rb') as xml_file:
                # https://github.com/python/typeshed/pull/3725
                xml.sax.parse(xml_file, handler)  # type: ignore
        except xml.sax.SAXException as exc:
            print(f'{filename}: Failed to xml parse ({exc})')
            retval = 1
    return retval


if __name__ == '__main__':
    exit(main())

import argparse
import xml.sax.handler
from typing import Optional
from typing import Sequence


def main(argv: Optional[Sequence[str]] = None) -> int:
    parser = argparse.ArgumentParser()
    parser.add_argument('filenames', nargs='*', help='XML filenames to check.')
    args = parser.parse_args(argv)

    retval = 0
    handler = xml.sax.handler.ContentHandler()
    for filename in args.filenames:
        try:
            with open(filename, 'rb') as xml_file:
                xml.sax.parse(xml_file, handler)
        except xml.sax.SAXException as exc:
            print(f'{filename}: Failed to xml parse ({exc})')
            retval = 1
    return retval


if __name__ == '__main__':
    exit(main())

Remove `type: ignore`