Diffoscope 156 throws an exception on a valid XML file with entities

Hello diffscope maintainers,

diffscope throws an exception when a valid XML file (with entities) is processed. (This issue might be related to #166)

Sample file: fontdata.xml from the Debian package khmerconverter found in https://packages.debian.org/buster/all/khmerconverter/filelist

$ diffoscope --version

diffoscope 156

I have the recommended package python3-defusedxml installed.

$ cd /usr/share/khmerconverter

$ diffoscope fontdata.xml fontdata.xml

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/diffoscope/main.py", line 747, in main
    sys.exit(run_diffoscope(parsed_args))
  File "/usr/lib/python3/dist-packages/diffoscope/main.py", line 701, in run_diffoscope
    difference = compare_root_paths(path1, path2)
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/compare.py", line 71, in compare_root_paths
    file1 = specialize(FilesystemFile(path1, container=container1))
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/specialize.py", line 55, in specialize
    if try_recognize(file, cls, cls.recognizes):
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/utils/specialize.py", line 38, in try_recognize
    if not recognizes(file):
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/xml.py", line 98, in recognizes
    file.parsed = _parse(f)
  File "/usr/lib/python3/dist-packages/diffoscope/comparators/xml.py", line 64, in _parse
    xml = minidom.parse(file)
  File "/usr/lib/python3/dist-packages/defusedxml/minidom.py", line 22, in parse
    return _expatbuilder.parse(
  File "/usr/lib/python3/dist-packages/defusedxml/expatbuilder.py", line 90, in parse
    result = builder.parseFile(file)
  File "/usr/lib/python3.8/xml/dom/expatbuilder.py", line 207, in parseFile
    parser.Parse(buffer, 0)
  File "../Modules/pyexpat.c", line 481, in EntityDecl
  File "/usr/lib/python3/dist-packages/defusedxml/expatbuilder.py", line 35, in defused_entity_decl
    raise EntitiesForbidden(name, value, base, sysid, pubid, notation_name)
defusedxml.common.EntitiesForbidden: EntitiesForbidden(name='mark', system_id=None, public_id=None)

The reason for the rejection of entities is explained on https://pypi.org/project/defusedxml/

When I remove the file extension, I get no exception:

cp fontdata.xml fontdata

diffoscope fontdata fontdata

Can the xml comparator fall back to the default text comparator in case an exception is thrown?

With kind regards, Roland Clobus