Refactor to use slackclient in place of slacker

[cekees]

In case it's useful... I replaced the calls to slacker with calls to slackclient and use the conversations API to https://python-slackclient.readthedocs.io/en/latest/conversations.html. I also found some minor string processing issues in python 3.8 on a mac.

[jonmchan]

This is indeed useful for getting around the deprecated API calls issue, but the export generated with this codebase no longer is compatible with slack-export-viewer.

 slack-export-viewer -z 20200816-195413-slack_export/
Archive already extracted. Viewing from slack-export/20200816-195413-slack_export...
Traceback (most recent call last):
  File "/usr/local/bin/slack-export-viewer", line 8, in <module>
    sys.exit(main())
  File "/Library/Python/2.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/Library/Python/2.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Library/Python/2.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/Library/Python/2.7/site-packages/slackviewer/main.py", line 61, in main
    configure_app(app, archive, channels, no_sidebar, no_external_references, debug)
  File "/Library/Python/2.7/site-packages/slackviewer/main.py", line 29, in configure_app
    top.mpim_users = reader.compile_mpim_users()
  File "/Library/Python/2.7/site-packages/slackviewer/reader.py", line 108, in compile_mpim_users
    mpim_members = {"name": mpim["name"], "users": [self.__USER_DATA[m] for m in mpim["members"]]}
KeyError: 'members'

Any ideas to fix this?