Update oauth_v2_access to include redirect_uri

[tomasreimers]

Summary

Following the docs here, we want to add the redirect URI as a named parameter: https://api.slack.com/legacy/oauth

Requirements (place an x in each [ ])

• 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.

[seratch]

A potential gotcha: while redirect_uri is optional, it is required if your app passed it as a parameter to oauth/authorization in the first step of the OAuth flow.

https://api.slack.com/methods/oauth.v2.access#arg_redirect_uri

As described in the above page, it's also possible to omit the redirect_uri parameter when starting OAuth flow. If this named arg is optional, I'm fine with this change for more developer-friendliness.

[seratch]

We have two endpoints as below. Applying the same to oauth.access is preferred for consistency.

• https://api.slack.com/methods/oauth.v2.access
• https://api.slack.com/methods/oauth.access

[tomasreimers]

Thank you @seratch , and apologies about being slow to update, work was a lot this month.

Went ahead and added it to both methods as well as making the param optional, lmk if that's what you were thinking :)

[seratch]

@tomasreimers Thanks - I've fixed minor warnings by flake8 and merged your changes!