onedrive backend uses 'unicode' constructor from python2.7 and fails

[Enno L]

The onedrive backend uses the 'unicode' constructor from Python2.7. This leads to:
 "NameError: name 'unicode' is not defined"

Duplicity version: 0.8.08
Python: Python 3.6.9 (default, Dec 8 2019, 01:17:00)
[GCC 4.2.1 Compatible FreeBSD Clang 6.0.1 (tags/RELEASE_601/final 335540)] on freebsd12
OS Distro and version: FreeBSD BSDServer 12.0-RELEASE-p10 FreeBSD 12.0-RELEASE-p10 GENERIC amd64
Type of target system: Onedrive

Error log:
Rückverfolgung des vorangegangenen Fehlers: Traceback (innermost last):
  File "/usr/local/lib/python3.6/site-packages/duplicity/backend.py", line 371, in inner_retry
    return fn(self, *args)
  File "/usr/local/lib/python3.6/site-packages/duplicity/backend.py", line 531, in put
    self.__do_put(source_path, remote_filename)
  File "/usr/local/lib/python3.6/site-packages/duplicity/backend.py", line 517, in __do_put
    self.backend._put(source_path, remote_filename)
  File "/usr/local/lib/python3.6/site-packages/duplicity/backends/onedrivebackend.py", line 262, in _put
    u'Content-Length': unicode(len(chunk)),
 NameError: name 'unicode' is not defined

[Enno L (ennol)]

A possible fix for this could be in onedrivebackend.py:

262c262
< u'Content-Length': unicode(len(chunk)),
---
> u'Content-Length': u'%d' % len(chunk),