[BUG] slack returner fail when `changes: True`

Description
slack returner don't work with
slack.changes or slack.only_show_failed

Setup
salt 3001.1 on master/minion on ubuntu16.04

Expected behavior
receive report on changes or failed states

Additional context

2020-09-04 08:52:09,266 [salt.minion      :1820][ERROR   ][2638] The return failed for job 20200904085127809008: unhashable type: 'dict'
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/salt/minion.py", line 1811, in _thread_return
    minion_instance.returners[returner_str](ret)
  File "/usr/lib/python3/dist-packages/salt/returners/slack_returner.py", line 218, in returner
    returns = {(key, value) for key, value in returns.items() if value['result'] is not True or value['changes']}
  File "/usr/lib/python3/dist-packages/salt/returners/slack_returner.py", line 218, in <setcomp>
    returns = {(key, value) for key, value in returns.items() if value['result'] is not True or value['changes']}
TypeError: unhashable type: 'dict'

Start in salt/returners/slack_returner.py at line 218, where filtering reports with changes or only_show_failed produces the unhashable dict error. Reproduce the returner behavior using the Salt 3001.1 setup and the reported options. Done means the Slack returner sends reports for changed or failed states without this error.