added a generic function of `months_observable` called `observable_interval` that takes 'days','weeks',or 'months' and returns the same results.

old version was not including the 31st of December in its calculations for `months_observable`.
This was the only function to use this, so it is an easy update, and with `observable_interval` added is needed.

Made `months_observable` an alias of `observable_interval` to maintain backwards compatibility. Also added `weeks_observable` and `days_observable` aliases.

I also made a small change to `observable_interval` to enhance output readability in the case a target is never visible given the constraints. (would have been "set()" now is "{}")

fixed the issues that were noted with the tox check

minor white space errors removed in aliased functions and between aliased function