Upgrade reqwest dependency

[briprowe]

The default reqwest client in newer versions of reqwest have asynchronous behavior.

Solution: Since blocking behavior is assumed by the rest of this crate, it's simplest to use reqwest's blocking client.

[dten]

You must use the required version (~0.9.6) or implement your own SlackWebRequestSender

[dten]

Thanks for the changes though. I was waiting for non alpha versions of reqwest and tungstenite before looking at async changes

[sts10]

Looks like reqwest just came out of alpha for async stuff with v0.10.0

Change default Client API to async. The previous blocking client API is [available] at reqwest::blocking.

[dten]

Typical 🤣