[ Chris Lamb ]
* Make two required adjustments for the new version of the src:file package:
  - file(1) version 5.46 now emits "XHTML document" for .xhtml files, such as
    those found nested within our .epub tests. Therefore, match this string
    when detecting XML files. This was causing an FTBFS due to inconsistent
    indentation in diffoscope's output.
  - Require the new, upcoming, version of file(1) for a quine-related
    testcase after adjusting the expected output. Previous versions of
    file(1) had a duplicated "last modified, last modified" string for some
    Zip archives that has now been removed.
* Add a missing subprocess import.
* Bump Standards-Version to 4.7.2.