deprecate utils.format_string

[davidism]

It appears to do exactly what string.Template does.

[northernSage]

I would be glad to help with this.

[davidism]

You're welcome to work on issues, no need to ask first. The deprecation warning should be something like 'format_string' is deprecated and will be removed in 2.1. Use 'string.Template' instead. There's a few places the function is used internally which should be replaced with string.Template, as well as the implementation itself.

[northernSage]

Okay, I will get to it and write a PR in the next few days.