This fixes the indexer debian package build.
```
=================================== FAILURES ===================================
________________________ test_compute_mimetype_encoding ________________________
def test_compute_mimetype_encoding():
"""Compute mimetype encoding should return results"""
for _input, _mimetype, _encoding in [
("du français".encode(), "text/plain", "utf-8"),
(b"def __init__(self):", "text/x-python", "us-ascii"),
(b"\xff\xfe\x00\x00\x00\x00\xff\xfe\xff\xff", "application/octet-stream", ""),
]:
actual_result = compute_mimetype_encoding(_input)
> assert actual_result == {"mimetype": _mimetype, "encoding": _encoding}
E AssertionError: assert {'encoding': ...cript.python'} == {'encoding': '...ext/x-python'}
E Omitting 1 identical items, use -vv to show
E Differing items:
E {'mimetype': 'text/x-script.python'} != {'mimetype': 'text/x-python'}
E Use -v to get the full diff
swh/indexer/tests/test_mimetype.py:36: AssertionError
```