diffoscope 166: Test suite errors with guix and python 3.8

The switch to magic.Magic causes test suite errors and failures building diffoscope 166 on guix using python 3.8. Full log attached.

qsi38wmgnkvs2lqx36n7p99540skr2-diffoscope-166.drv.bz2

Reverting commit c72c30f2 works around the issue.

Most of the errors are similar to the following:

============================= test session starts ==============================
platform linux -- Python 3.8.2, pytest-5.3.5, py-1.8.1, pluggy-0.13.1
rootdir: /tmp/guix-build-diffoscope-166.drv-0/source, inifile: pytest.ini
collected 462 items

...

tests/comparators/test_rpm.py:93: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
diffoscope/comparators/utils/specialize.py:54: in specialize
    if try_recognize(file, cls, cls.recognizes):
diffoscope/comparators/utils/specialize.py:37: in try_recognize
    if not recognizes(file):
diffoscope/comparators/utils/file.py:154: in recognizes
    file.magic_file_type,
diffoscope/comparators/utils/file.py:308: in magic_file_type
    self._magic_file_type = File.guess_file_type(self.path)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cls = <class 'diffoscope.comparators.utils.file.File'>
path = '/tmp/guix-build-diffoscope-166.drv-0/source/tests/data/test1.rpm'

    @classmethod
    def guess_file_type(cls, path):
        if not hasattr(cls, "_mimedb"):
>           cls._mimedb = magic.Magic()
E           TypeError: __init__() missing 1 required positional argument: 'ms'

diffoscope/comparators/utils/file.py:73: TypeError
============ 43 failed, 35 passed, 58 skipped, 326 errors in 39.26s ============
command "python" "-c" "import setuptools, tokenize;__file__='setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\\r\\n', '\\n');f.close();exec(compile(code, __file__, 'exec'))" "test" failed with status 1