Deprecated method im.open

[ggeneralov]

Describe the bug

Method im.open is deprecated and should be replaced by conversations.open

https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api

What type of issue is this? (place an x in one of the [ ])

• bug

Requirements (place an x in each of the [ ])**

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.