HMAC SHA256 signatures in Python and Flask.

I'm playing around a bit with the Slack API, which I'll have a longer post on in a bit. One part of the integration requires generating an HMAC SHA256 signature to verify requests are from Slack. There weren't too many helpful search results, and some of them like the hmac module docs don't include examples. Here are some quick notes for folks in future attempting the same thing.

h/t to Joe Kampschmidt's post which covers signing well.

First step is to instrument your test application is capturing the full headers an...