Before the changes from a19e2831
overrides for source packages were created with "Priority: source" and
eventually updated to "Priority: optional".  This lead to a change in
the "Sources" index and a diff to be created.

Now source overrides are created with "Priority: optional" and this
change no longer happens.  The diff file disappearing is therefore
okay.

`IOError` was made an alias for `OSError` in Python 3.3.

The old name is confusing as Python 2's `raw_input` is gone.

The check for either `--carbon-copy` or `--done` is performed a bit
later with a correct error message if neither is specified:

  Need a -C/--carbon-copy if not closing a bug and not doing a cruft removal.

The check removed here originally checked that `--carbon-copy` was
always used together with `--done` (as the error message implied), but
the condition was unintentionally broken in
84bb4881.

Use of `--carbon-copy` without `--done` seems useful and desired (per
the error above), so just remove the incorrect check here.

In addition the HTML output mode should be fixed which previously
tried to write `bytes` to `sys.stdout` in the `main` function.

The arguments returned by `apt_pkg.parse_commandline` are already of
type `str`. Various subcommands also assumed so.

This change removes the conversion to `str` from the remaining
subcommands. With this the `six` module is also no longer needed.

The `name` comes from a regexp match on a `str` and is therefore
already a Unicode `str`.

Already remove the now unused `force_to_utf8` function.

Both `TarFile` and access to the filesystem via `UnpackedSource`
decode filenames. Therefore `name` is already a (Unicode) `str` since
the switch to Python 3.

Note that this imples dak currently no longer handles non-UTF-8
filenames in uploaded binary or source packages.

Packages which go into byzantium should still go via landing, but we
need to handle the updates suites as well now, to stay in sync with
Debian.

Expliclty inheriting from `object` was needed in Python 2 for
new-style classes, but is no longer needed in Python 3 where all
classes implicitly inherit from `object`.