AttributeError during backup (lp:#1833447)

[Original report](https://bugs.launchpad.net/bugs/1833447) created by **Jacob Mansfield (cyberjacob)**
```
Duplicity Version: 0.8
Python Version: 3.5.3
OS Distro: Debian
OS Version: Stable (Stretch, 9.9)
Source: Linux filesystem (ext4)
Destination: Backblaze B2
Command line:  /usr/local/bin/duplicity -v9 --full-if-older-than 14D
--exclude /**.DS_Store --exclude /**Icon? --exclude /**.AppleDouble
--include=/mnt/Pictures/ --include=/home/ --include=/mnt/Documents/
--include=/mnt/Usershare/ --include=/root/ --exclude=** /
b2://xxxx:xxxx@xxxx/

Logs:
[Removed lots of files being checked]
Releasing lockfile
b'/root/.cache/duplicity/09cf580279539188df4bbf911b749576/lockfile'
Removing still remembered temporary file
/tmp/duplicity-4u664jq5-tempdir/mkstemp-c0yaehv4-1
Removing still remembered temporary file
/tmp/duplicity-4u664jq5-tempdir/mktemp-w_t_j867-4
Releasing lockfile
b'/root/.cache/duplicity/09cf580279539188df4bbf911b749576/lockfile'
Traceback (innermost last):
  File "/usr/local/bin/duplicity", line 1706, in <module>
    with_tempdir(main)
  File "/usr/local/bin/duplicity", line 1692, in with_tempdir
    fn()
  File "/usr/local/bin/duplicity", line 1538, in main
    do_backup(action)
  File "/usr/local/bin/duplicity", line 1662, in do_backup
    full_backup(col_stats)
  File "/usr/local/bin/duplicity", line 568, in full_backup
    globals.backend)
  File "/usr/local/bin/duplicity", line 425, in write_multivol
    globals.volsize)
  File "/usr/local/lib/python3.5/dist-packages/duplicity/gpg.py", line 393,
in GPGWriteFile
    data = block_iter.__next__().data
  File "/usr/local/lib/python3.5/dist-packages/duplicity/diffdir.py", line
543, in __next__
    result = self.process(next(self.input_iter))  # pylint:
disable=assignment-from-no-return
  File "/usr/local/lib/python3.5/dist-packages/duplicity/diffdir.py", line
681, in process
    data, last_block = self.get_data_block(fp)
  File "/usr/local/lib/python3.5/dist-packages/duplicity/diffdir.py", line
710, in get_data_block
    if fp.close():
  File "/usr/local/lib/python3.5/dist-packages/duplicity/diffdir.py", line
460, in close
    self.callback(self.sig_gen.getsig(), *self.extra_args)
  File "/usr/local/lib/python3.5/dist-packages/duplicity/diffdir.py", line
141, in callback
    sigTarFile.addfile(ti, io.BytesIO(sig_string))
  File "/usr/lib/python3.5/tarfile.py", line 1962, in addfile
    buf = tarinfo.tobuf(self.format, self.encoding, self.errors)
  File "/usr/lib/python3.5/tarfile.py", line 804, in tobuf
    return self.create_gnu_header(info, encoding, errors)
  File "/usr/lib/python3.5/tarfile.py", line 835, in create_gnu_header
    return buf + self._create_header(info, GNU_FORMAT, encoding, errors)
  File "/usr/lib/python3.5/tarfile.py", line 925, in _create_header
    stn(info.get("gname", ""), 32, encoding, errors),
  File "/usr/lib/python3.5/tarfile.py", line 156, in stn
    s = s.encode(encoding, errors)
 AttributeError: 'bytes' object has no attribute 'encode'

Releasing lockfile
b'/root/.cache/duplicity/09cf580279539188df4bbf911b749576/lockfile'
Exception ignored in: <function
WeakValueDictionary.__init__.<locals>.remove at 0x7efed9f0d840>
Traceback (most recent call last):
  File "/usr/lib/python3.5/weakref.py", line 117, in remove
TypeError: 'NoneType' object is not callable
```

Story points: 6.0