* Temporary directory handling:
  - Ensure we cleanup our temporary directory by avoiding confusion between
    the TemporaryDirectory instance and the underlying directory.
    (Closes: #981123)
  - Use a potentially-useful suffix to our temporary directory based on the
    command-line passed to diffoscope.
  - Fix some tempfile/weakref interaction in Python 3.7 (ie. Debian buster).
    (Closes: reproducible-builds/diffoscope#239)
  - If our temporary directory does not exist anymore (eg. it has been
    cleaned up in tests, signal handling or reference counting),  make sure
    we recreate it.

* Bug fixes:
  - Don't rely on magic.Magic(...) to have an identical API between file's
    magic.py and PyPI's "python-magic" library.
    (Closes: reproducible-builds/diffoscope#238)
  - Don't rely on dumpimage returning an appropriate exit code; check that
    the file actually exists after we call it.

* Codebase changes:
  - Set a default Config.extended_filesystem_attributes.
  - Drop unused Config.acl and Config.xattr attributes.
  - Tidy imports in diffoscope/comparators/fit.py.

* Tests:
  - Add u-boot-tools to test dependencies so that salsa.debian.org pipelines
    actually test the new FIT comparator.
  - Strip newlines when determining Black version to avoid "requires black
    >= 20.8b1 (18.9b0\n detected)" in test output (NB. embedded newline).
  - Gnumeric is back in testing so re-add to test dependencies.
  - Use assert_diff (over get_data, etc.) in the FIT and APK comparators.
  - Mark test_apk.py::test_android_manifest as being allowed to fail for now.
  - Fix the FIT tests in buster and unstable.